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Thread started 20 Feb 2018 (Tuesday) 11:36

# Calculations on magnification and focus

Feb 20, 2018 11:36 |  #1

I understand that the magnification of the specimen onto the sensor/film is the result of the ratio between the distance from the lens to the sensor divided by the distance between the specimen and the lens.
But as the above are reached, one needs to alter one or both to enable focus and that depends on the focal length of the lens in use.

What is the effect of the lens' focal length in these calculations?

To put it in another way: If I want to have 5:1 magnification ratio, I need to have 1cm distance from the specimen and 5cm distance from the sensor. This doesn't work as simple as that. These figures may not give a focused image. So these need to be adjusted, keeping the 5:1 ratio, until focus is achieved. Perhaps at 1.5cm distance from the specimen and 7.5cm distance to the sensor. I suspect this depends on the focal length of the lens, but haven't found how the lens affects this so I can decide what's the right focal length to achieve the desired magnification when other factors need to be observed (e.g. a given bellows length with the slide copier attachment mounted at the one end of the bellows.

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Feb 20, 2018 11:55 |  #2

Using a macro calculation program I find that

• using 50mm lens with 100mm extension, when focused at 0.2m the magnificaiton is 3:1 (3x)
• using using 100mm lens with 100mm extension, when focused at 0.2m the magnificaiton is 3:1 (3x)
• using 50mm lens with 100mm extension, when focused at 0.1m the magnificaiton is 5:1 (5x)
• using 100mm lens with 100mm extension, when focused at 0.1m the magnificaiton is 5:1 (5x)

so you see the effect of lens FL has no apparent effect!

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Feb 20, 2018 17:25 |  #3

SaSi, have a look at the "Thin Lens" model - there are loads of descriptions on the net, but Wikipedia's isn't particularly clear, I think.

If you use a longer FL lens, the distances BOTH sides of the lens increase, for the same magnification.

So for 1:1,
with a 50mm lens you'd have 100mm subject-lens and lens-sensor distances,
with a 100mm lens you'd have 200mm subject-lens and lens-sensor distances.

Real lenses aren't "thin" though, the geometry is changed. Often they aren't symmetrical either, the manufacturer thus makes the "working" distances each side of the lens, whatever's easier to use. EG a telephoto lens is shorter than the FL, and a wideangle like a 20mm on a full frame SLR is always much further than that from the sensor, which is because it's a retrofocus design. Otherwise the mirror would be in the way.

Th upshot is that calculations are difficult!

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Feb 20, 2018 18:00 as a reply to  @ Chris.R's post |  #4

While the distances on each side of the lens are 2*FL when the magnification is 1:1, I have never seen any equation for higher magnifications.
So we know that shooting 1:1 with 50mm lens needs 4*50mm as the subject-to-focal-plane distance, and we know that shooting 1:1 with 150mm lens needs 4*150mm as the subject-to-focal-plane distance.

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Calculations on magnification and focus
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 x 1600 y 1600

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