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02-06-2021, 02:02 AM - 1 Like   #1
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Choices of focal length vs aspect ratios

The crop factor between "full frame" (24x36) and cropped digital medium format (33x44) is 0.79 is based on the ratio of the diagonals (please correct me if I'm wrong here). But the aspect ratio being 3:2 for 24x36 and 4:3 for 33x44, is it aesthetically correct to use that 0.79 crop factor to know what focal length in MF equate another focal length in 24x36? Would you favor the ratio of verticals (=0.72) or the ratio of horizontals (0.82) for the equivalence?

02-06-2021, 04:19 AM - 2 Likes   #2
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This is a very interesting question - and depends how you're going to crop the 3:2 or 4:3 image.

Say you make 1:1 crops from each - I'd go with with the 0.72 as it's the vertical one that will ultimately limit the size of the square.

If you make 2:1 or 16:9 crops from each I'd use the 0.82 factor.

If it's a 4:3 crop being taken from the 3:2 I'd go with the diagonal.

I wouldn't worry too much about it - composition is about field of view - not it's equivalent to one sensor/film size to another. Go with whatever looks right in the viewfinder and be done
02-06-2021, 04:46 AM   #3
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Maybe the focal length equivalent on full frame / 24x36 would feel a little wider, due to the 3:2 aspect ratio, and 4:3 being closer to a square than 3:2 is?
It's an important question when selecting a lens on MF coming from FF or FF coming from MF. Perhaps for normal and tele lenses it doesn't make a big difference, but for a wide angle lens a few mm difference is significant.
02-06-2021, 07:13 AM   #4
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QuoteOriginally posted by biz-engineer Quote
Maybe the focal length equivalent on full frame / 24x36 would feel a little wider, due to the 3:2 aspect ratio, and 4:3 being closer to a square than 3:2 is?
It's an important question when selecting a lens on MF coming from FF or FF coming from MF. Perhaps for normal and tele lenses it doesn't make a big difference, but for a wide angle lens a few mm difference is significant.
It is - my 28-45 is 22mm 35mm equivalent at 28mm - except it won't appear as wide because of the aspect ratio.

As it was I cropped my 3:2 files a lot to 4:3, or 5:4 or even 1:1 or 2:1 and 16:9.

3:2 is an odd aspect ratio - not wide enough to be panoramic or cinematic - too skinny on a 2:3 portrait orientation with 5:7, 3:4 and 4:5 all are less skinny and more natural on the eye. I also found 3:2 does get quite close to the edge of the image circle - so mushier at the sides. It's not a format I will miss.

Quite frankly camera makes should just make sensors square and let us crop them as we see fit - and no more mush corners or vignetting.

02-06-2021, 09:01 AM   #5
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They should make the sensor a circle where you can rotate and then crop your aspect of choice.
02-06-2021, 09:07 AM   #6
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QuoteOriginally posted by tuco Quote
They should make the sensors round
Good for fish-eye lenses!
02-06-2021, 05:08 PM - 1 Like   #7
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QuoteOriginally posted by tuco Quote
They should make the sensor a circle where you can rotate and then crop your aspect of choice.
That would be expensive since a lot of the semiconductor wafer would go to waste.
02-06-2021, 05:52 PM   #8
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QuoteOriginally posted by Wasp Quote
That would be expensive since a lot of the semiconductor wafer would go to waste.
So you pay extra for a camera that doesn't need to be held level.

02-06-2021, 07:28 PM   #9
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I would see what aspect ratio you tend to shoot/crop to? As long as it is narrower (wider horizontally I mean) than the medium format pick a lens that gets you the same horizontal FOV. Since the medium format is closer to square you will have excess in the vertical direction, which you likely will crop.

Also I find matching the horizontal works for me even when the new format will have a smaller FOV in the vertical direction. As an example I use a finder that get me 120 degree FOV horizontally and only about 75 degree vertically, and yet it is fine for a wide angle lens on a 4:3 format. I lose seeing some on top and bottom but it seems fairly easy for me to instinctively adjust for this.**
_____
** Sorry this does not address your question. Anyway if you use the horizontal FOV you will not have to worry that you are missing something as the lens will be the widest FL. But likely your gut assessment is best for you.

Last edited by dms; 02-06-2021 at 08:19 PM.
02-07-2021, 06:41 AM - 2 Likes   #10
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QuoteOriginally posted by dms Quote
Also I find matching the horizontal works for me even when the new format will have a smaller FOV in the vertical direction. As an example I use a finder that get me 120 degree FOV horizontally and only about 75 degree vertically, and yet it is fine for a wide angle lens on a 4:3 format. I lose seeing some on top and bottom but it seems fairly easy for me to instinctively adjust for this.**
That's a good input. After some further thoughts, I'd say for myself that it is better to consider the respective widths of sensors for wide angle lenses, and consider the height of sensors for tele and super tele lenses.
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