A few scanning tips


A Calculator About Copying Slides with a Camera

The digital camera is a very fast way to scan many slides (digitize them, into a file for computer use), see Copying thousands of slides. A 12 to 24 megapixel camera (typically a DSLR) with 1:1 macro lens is quite capable of getting the all the resolution that a normal 35 mm slide has to give. The camera must accept interchangeable lenses, and the slide is lighted from behind. A macro lens is very simple to use, it easily focuses at any distance same as any regular lens. Extension tubes are relatively difficult to use, you assemble the specific setup which then mostly focuses at only one distance and magnification. But using a macro lens, or extension tubes with a proper lens, even a small megapixel camera can create an copy very suitable for a HDTV or monitor. Best case is enough slide copy magnification allowing one side of the film to fill the frame (slides smaller than the camera sensor will not fill the frame at 1:1). The best film scanners were 4000 dpi, and it was debated then that 3000 dpi was plenty to resolve maximum film detail. 12 to 24 megapixels will be ideal to do that, and the camera macro lens is superb quality.

35 mm slides are the most common interest, but the calculator does a wide range of many film sizes, sensors, lens ratios and crop factors, with macro lens or extension tubes. You specify the lens situation you have, and it will tell you what it can do, and/or what is needed to do it. The image size needed or possible for the situation will be computed.

This calculator computes the expected results that your camera and lens can do, computing the macro magnification, and the maximum image size result, and also its equivalent scan resolution. The slide must be mounted at the distance where focus and magnification are calculated. The calculator does need to know your accurate sensor size, and it does generally expect macro lens capability or suitable extension tubes. Note that even at 1:1 magnification, copying film smaller than the sensor size cannot fill the frame, so crop will be required. Or if sensor is smaller than the film, then less than 1:1 magnification is required.

Slide Copy Calculator

Compute Macro Camera Scan to copy slides

 The camera is   Digital   or   Film

Three choices to specify sensor size
1 Sensor Size × mm
Sensor Pixels × pix
2 Sensor Pixels × pix
Crop Factor x
3 Megapixels
Aspect Ratio
as 4:3 or 1.333
Crop Factor x
Camera Lens
Extension tube, using:
mm Focal Length Lens
xMagnification at Closest focus (from lens spec)
mm Extension Tube length
Known magnification: x
Known reproduction ratio:
Copying Image size: %
Printing resolution
Any size
   scan area
× inch

Image Size: If helpful, you can specify a smaller copied image size goal, to compute magnification of an image size reduced to maybe like 95% of the camera frame size, so that fitting it into the exact camera frame is not so critical (to reduce probable chance of cutting off some of it). For example, entering 95 (meaning reduction to 95% size) if 4000 pixels will reduce the image size to (4000 x 0.95) = 3800, or 200 pixels smaller (than the 4000 pixel camera sensor size). Recommended, since we likely have lots of pixels,and since aspect factor differences likely require cropping anyway, and it sure makes it much easier.

Sensor Option 1 (sensor dimensions): Use that if you know it. Probably knowable in camera specs if it uses interchangeable lenses. Sensor size for compacts and cell phones is much more nebulous, but see more detail of the sensor size issues.

Sensor Option 3 (megapixels): Megapixels and Aspect Ratio specifications are normally approximate rounded numbers which blurs computing results. Megapixel specs may include other invisible pixels used for other purposes. We do obviously know the precise pixel dimensions of our camera, so you can multiply camera pixels (width × height) / 1,000,000 to get exact megapixels. Dividing camera pixels (width / height) will get exact aspect ratio. Sensor option 2 can do that for you, and this can make Sensor Option 2 or 3 be pretty close if we know precise crop factor. Option 1 will compute crop factor exactly if you know sensor size in mm, but sensor size specs are also rounded numbers, perhaps three significant digits. Crop factor can be computed from Equivalent Focal Length numbers, again see more detail.

Aspect Ratio can be entered as formats like 4:3 or 1.3333:1 or just 1.3333 works here (assumes the :1). The latter format allows more precise exact values, not rounded. Option 3 rounds computed pixel dimensions to the nearest multiple of 8 because the cameras do that, which can change the aspect ratio slightly (pixels are sometimes rounded differently unless precise megapixels and aspect ratio are specified). But crop and resample operations of photo editors do Not observe multiples of 8. And to always match those four digits of pixel size will require at least four and better five significant digits of megapixels, aspect ration and crop factor (Option 2 will show those digits except crop factor).

Lens Option 1, (macro lens). The macro lens is the easiest and overwhelmingly most convenient solution, because it works like any regular lens, meaning it easily focuses at any distance and magnification up to its maximum, and is also highly corrected for that extreme. Macro lenses are easy without any calculator, but this calculator does tell the expected result, and offers the appropriate lens setting for the scale needed. Then adjust the slide distance to where it is in focus there, and it fills the frame as desired. Good bets are that a macro lens may become one of your favorite lenses, and that extension tubes may qualify for the “never again” category. 😊

Lens Option 2, (extension tube). Extension tubes are relatively difficult to use, and beginners may be shocked to find out lenses really don’t focus on an extension tube at high magnification. Or rather, they pretty much focus at some one distance then, and you move the camera or subject back and forth to find where that focused distance is. With camera on a tripod, it's usually easier to find focus by moving the subject back and forth. Specify the actual real focal length that is marked on the lens, NEVER any “Effective Focal Length”. A zoom lens can help to adjust result size, but zoom lenses on an extension tube are notoriously Not the sharpest quality. The calculator initial defaults assume lens is set to closest focus. The lens is typically used at its closest focus setting for maximum magnification, but the Infinity focus setting does offer another (lower) magnification choice. Maximum Magnification at Closest Focus is typically found in the lens specifications (may be called Maximum Magnification, at corresponding Minimum Focus distance). Focal Length and Magnification at Closest Focus are rounded specifications.

If you don’t know the lens spec for Maximum Magnification at Closest Focus, you can enter 0 for Closest Magnification and use the lens at its Infinity focus setting. But much better, see how to measure magnification (next page). Or the B&H product pages are a very good place to find the specifications of current lenses. Since our available choices of lenses and extension tubes are limited, do realize that the two Infinity and Closest focus settings do offer some choice of magnification. When on extension tubes at higher magnification, lens focus has almost zero effect, but it does have mild magnification range. Actual focus must be found by changing subject to lens distance until it is in focus.

So if using extension tubes, enter the correct Maximum Magnification at Closest Focus Distance (from the lens specs, or you can measure it), which entering it as non-zero implies the lens is used at closest focus. Then if that maximum magnification is too large for the slide size, it will also show the lesser Infinity focus magnification. And if the Needed magnification is between the Minimum and Maximum possible (between Infinity and Closest focus), then you should be able to find the best intermediate focus setting and position. Or if that focus range cannot work, it will show what extension and magnification is instead necessary. Or if the closest magnification is too small for the slide to fill the frame, it will show what could be accomplished anyway, which might be a satisfactory goal for the purpose. Extracting copied slides likely will likely need a little crop anyway, perhaps even for the purpose of improving pictorial content too.

To indicate using the lens with focus at Infinity, specify the Magnification at Closest Focus Distance to be 0 (which won’t mean zero magnification at macro distance, but actual infinity distance technically is zero magnification, see examples at the extension tube formulas on next page), which will then compute magnification for the lens at infinity focus on the extension tube. Except the Infinity mode won’t know the closest focus spec to be able to compute the larger possibility.

Lens Option 3 and 4 (known magnification): This could be useful for your extension tube setup. To determine the magnification of your setup, see how to measure magnification on next page.

Rounding: If you know precise sensor size in mm and pixels, then great, that should be used in Option 1. Otherwise, pixel dimensions are typically 4 significant digits. Megapixels and crop factor and aspect ratio are rounded specifications. To recompute those four exact digits of pixel dimensions in Option 2 or 3 likely requires Megapixels and Crop Factor and Aspect Ratio to also be accurate to four significant digits because in math, the final answer can only contain as many significant digits as the least precise value. So the computed pixel dimensions could be just a few pixels off, but a few tenths of a percent will certainly be close enough for practical purposes.

Negative film: Mounting the film strip to hold it will be more difficult, but B&W film copies easily, and is inverted simply using the photo editors Invert menu. Then see a suggestion about improving grayscale contrast. Color negative film is much more difficult due to the orange mask, which is deep blue when inverted. There are ways to remove that digitally, but the best way is to use a real film scanner for color negatives, which has better analog methods to remove it (which cannot clip the tones). See correcting color negatives.

For slide copy photography, the slide should be lighted from the back side, through the film. See more detail about the process.

Focus with Macro lenses: Macro lenses are very popular, because they are extremely convenient to use. They focus easily like any lens at any supported magnification from infinity to closest macro range, which is often 1:1 at closest focus. They are also designed for the exact macro purpose, whereas extension tubes are an added make-shift on a different design. At 1:1 reproduction ratio, the distance between lens and slide will be roughly the marked focal length, or technically a bit less. If setting a specific ratio like 1:1, the focus ring on the lens simply selects that reproduction ratio. The macro lens will focus normally and easily at any distance, but to maintain that selected ratio, then focus at setup is adjusted by moving the slide slightly back and forth, to the place where it is in focus at that setting. The macro lens is more expensive, but IMO a day and night better choice, and greatly more useful, and may become your favorite lens for many things. There may be used older lenses that might be inexpensive and still be compatible with some current cameras. See Copying thousands of slides. The ES-1 attachment featured there is not required, but it’s about general slide copying with a camera.

Focus with Extension tubes: Extension tubes focus at only one distance, and then lens focus rotation hardly affects focus on an extension tube. Instead we have to move the subject or camera back and forth to find the one location where it will focus, which might not be suitable for your goal. A different magnification requires a different extension and/or a different lens focal length. A 20 mm extension on a 200 mm lens is not great effect, but it is huge on a 20 mm lens, so extension tubes affect shorter lenses greatly more than longer lenses.

If an extension tube does not provide the modern electrical lens contacts, your camera cannot control the lens on the extension. That may include inability to focus, and also no aperture stop-down. Kenko is one popular brand that provides the electrical contacts.

Specifications of individual regular prime lenses normally specify its native Maximum Magnification at closest focus distance, like perhaps 0.15x for a 50 mm lens at maybe 1.5 feet minimum (specified without extension tube). The extension tube effect then adds to this native magnification. The reproduction ratio is 1/magnification.

Focus Distance; Most camera interchangeable lenses today (maybe not movie lenses) show specifications for the "Minimum focus distance" (the closest focus at a few feet), and specs also specify the maximum magnification at that closest point. That is surely accurate from the manufacturer, but a problem with focal length with extension tube and/or magnification work is that they are based on the Thin Lens Equation, which simplifies to refer to a single element lens (like a simple magnifying lens). Camera lenses however are “thick lenses” composed of several elements, and the focal nodes are not necessarily in the “center” of a complex lens. Computing focus distance or focal length can differ maybe a couple of inches inside the lens (and internal focus extension is another factor). The error is not very significant at normal distances of several feet (so calculations are reasonable for Field of View or Depth of Field with normal lenses), but a couple of inches of focus distance is excessive error at macro distances, so it is not attempted here for that reason. Macro work computes with magnification instead.

Note that Focus Distance, including the spec for Closest Focus Distance (for maximum magnification), is measured to the sensor plane, which is very near the rear of the camera (location normally marked with a θ symbol). Macro calculations are concerned with Working Distance, which is distance to the front lens node (the focus node is usually inside the lens body somewhere). The magnification calculations are usually pretty close, and should be close enough, but distance may not measure to the same place you might expect. For that reason, macro focus distance is at best experimental, since calculation doesn’t know where the node is. But 1:1 macro focus distance to front lens element will often be somewhat less than the focal length.

Both macro lens or extension tube:

Copying slides is a mostly flat field, which macro lenses are designed for, and only stopping down to f/11 may be good enough. But extension tubes on regular lenses are less sharp in the corners, so at macro distances, maximum depth and sharpness will likely require stopping down quite far. Experiment with your setup to know what’s best, but normal use then is up towards maximum f/stop number. This exposure slows shutter speed, so motion becomes an blurring issue. Use of a tripod can help much to freeze shake motion (and don’t jiggle the camera with your shutter finger, use self timer or a cable release). And use of a speedlight flash helps also, including subject motion in other macro.

The focal length marked on a lens applies when it is focused at infinity, and the focal length becomes longer when focused up close (called internal extension). This focal length begins as a rounded approximation, and is not always exactly a precise or known number, which affects calculations more than it affects use.

To focus closer than infinity, the lens, or some front part of it, is extended forward to increase the actual focused distance to sensor plane (an effective focal length, longer than when at infinity). In many older lenses, the front lens group is extended forward to focus closer. But newer lenses (especially zooms) are often "internal focusing", where some of the internal elements are realigned to increase the focal length. See a diagram showing the simple geometry of the classic Thin Lens Equation, which is fundamental to lens optics. One significance of the diagram is that magnification = (Sensor dimension / field dimension), and also magnification = (focal length / distance to that field). In the special case of 1:1, the distances in front and in back of the lens are necessarily equal (because at 1:1, the real object size and the image object size are by definition (of 1:1) equal too, and they are the same ratio). But measuring distances at macro range is a real problem, because we never know where the node points are located that it measures to. So macro normally works with magnifications instead of distance.

Light loss:   is the theoretical light reduction at extreme close distances. It is Not actually a “loss”, the effective f/stop number simply increases at very close distance (because f/number is focal length / aperture diameter, and focal length is increased to focus up close). Through-the-lens metering automatically sees and measures this, but a hand-held meter does not. Some modern macro lenses (at least Nikon) report the new effective f/stop number to the camera, accounting for this reduction (important if using a hand-held meter). An f/stop number (like f/2.8) is the Focal Length / aperture diameter (the "effective" diameter as seen from in front of lens). Nominal f/2.8 means that at infinity focus, the focal length is 2.8x larger than the aperture diameter (precisely, 2.828x). Because when focused closer, the extension increases the focal length (to the sensor plane), which increases the actual f/stop number, not still f/2.8.

The maximum magnification for regular lenses is usually limited to about 0.1x to 0.2x (Maximum Magnification at Closest Focus), because that is roughly 1/4 to 1/2 EV loss, which could be noticeable, which is one reason they won't regularly focus much closer, to keep the loss minimal. However, at macro extensions (at focus distances of only a few inches), the difference increases significantly. The light loss is theoretically -2 EV at 1:1 due to this extension. However, modern macro lenses can internally decrease focal length at macro distances (and still focus up close), and the current Nikon macro lenses are typically more like -1.5 EV at 1:1 (f/2.8 macro lens reports f/4.8 at 1:1 instead of f/5.6, because the focal length is made shorter). So yes, there are exceptions to the computed light loss value or working distance (normally -2 EV at 1:1). Some macro lenses now automatically recompute the extended f/stop number, and show the updated number in the camera. If the camera meters through the lens, it sees the reduced light and just meters it normally. The use of a hand held light meter has to account for this manually, the 1:1 camera must be set to more exposure than manually metered.

Crop factor does Not affect subject magnification at 1:1, because 1:1 is simply already 1:1 from the lens. Cropping does affect frame size, but not object size. So there might be two ways to debate it in macro. Using the lens at 1:1 ratio, we do get 1:1, and the subject is the same 1:1 size on any size sensor, but a smaller sensor does crop the overall frame smaller, which is important if copying slides. The point is that 1:1 is 1:1, so that magnification is the same in any sensor, regardless. But yes, when the purpose is to copy the exact frame (like a slide) onto another size frame (like in the copy camera), then the ratio of the frame sizes is important.


Extension Tube Formulas (next page)

Determine magnification factor (next page)

Menu of the other photo pages here

Copyright © 2017-2024 by Wayne Fulton - All rights are reserved.

Previous Main Next