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01-06-2013, 07:35 PM   #1
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FoV, focal length and true aperture of lenses

In order to stimulate more technical, intellectual discussion on the optical properties of lenses on different formats, I have started this thread stemming from the initial discussions here: https://www.pentaxforums.com/forums/general-photography-industry/209882-next-...pentax-16.html

Now, aperture on a lens, let's say a FF lens like the FA 31mm f/1.8, should theoretically be constant between all the formats used to shoot with this particular lens. i.e. an f/5.6 setting on the lens should mean f/5.6 despite using the lens on an APS-C, Q or FF format. The crop factor alters the apparent aperture according to the FoV, increasing DoF with smaller formats than FF (considering the crop factor of FF is 1.0). But the actual aperture of the lens remains the same.

Is there any more to the optical qualities of the lens than this, or does the different sized formats actually somehow change the lens's characteristics?


Last edited by Ash; 01-06-2013 at 07:54 PM. Reason: spelling
01-06-2013, 08:15 PM   #2
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Don't know if this what you mean, but wouldn't a lens with good center sharpness but poor edge sharpness have different characteristics on different formats, especially the Q?
01-06-2013, 08:53 PM   #3
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With smaller formats and subsequently more cropping of the image circle from the lens, there will surely be a more consistent optimal IQ coming from the lens.
With good lenses such as the FA Limiteds, there isn't that concern.
01-06-2013, 08:55 PM   #4
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QuoteOriginally posted by Ash Quote
The crop factor alters the apparent aperture according to the FoV, increasing DoF with smaller formats than FF (considering the crop factor of FF is 1.0).
I'm not following. The only thing that's changing between formats is the cropping of the FoV. Everything else stays the same.

01-06-2013, 09:02 PM   #5
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Yes, I appreciate that, Ikarus.
I'm following on from the discussion in the other thread:

QuoteOriginally posted by Fogel70 Quote
Originally posted by Blue:
I didn't say anything about the FOV. The reason the FOV changes, all other variables kept the same is because the sensor size or film area changes. I specifically don't get into the equivalency with f values because it creates way too much confusion. An f2.8 lens always acts like an f2.8 lens regardless if it is a m4/3 sensor, aps-c sensor (Canon or Pentax/Nikon), full frame even thought the angle of view changes. This is because neither the physical focal length nor the physical aperture change.

How about if using a "inverted TC" to turn a FF lens into a APS-C lens?

With a 50/1.4 FF lens used with a inverted 1.4x TC you will get a equivalence of APS-C 35/1.0.
QuoteOriginally posted by Class A Quote
Originally posted by Blue
Only in "equivalency" terms. Otherwise the FA 77/1.8 LTD wouldn't be f1.8 on my MZ-3 and K-5. Physically, the focal length and f numbers are the same.

It is true that the focal length of the lens doesn't change (upon format change). But its FOV does.
It is also true that the f-ratio of a lens doesn't change (upon format change). But the total amount of light collected does.

Only FOV and total amount of light collected matter for the image.

Even on a Q, the F77/1.8 would be an f/1.8 lens. But the images you take on a Q with "f/1.8" correspond to "f/10" images on an FF. The DOF and noise of the Q-f/1.8 image and the FF-f/10 image would be the same.

One can call a Q-f/1.8 image an "f/1.8" exposure and one can call the Pentax 16-50/2.8 an f/2.8 lens. There is nothing wrong about this. Except that one has to bear the format in mind, according to which these figures are expressed. When expressed in the context of the Q, "f/1.8" means "very, very slow" (not "rather fast"). When expressed in the context of APS-C, "f/2.8" means "a stop slower than f/2.8 on FF". So if a TC converts the 16-50/2.8 into a 24-75/4.2, it doesn't make it slower (on an FF camera). It just translates the APS-C parameters into FF parameters.

I think we agree, but I thought it'd be worth spelling all those things out.
QuoteOriginally posted by ElJamoquio Quote
Originally posted by Ash
Semantics guys.


I'm not sure it is. If everyone understands the concept that a 1.55x TC will take a APS-C lens, give it the same DOF + SNR on FF as it had on the APS-C, but with possibly better resolving power... then yes, it's semantics.

I'm just not convinced that everyone understands the concept.
01-06-2013, 09:04 PM   #6
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I suggest Pentax/Ricoh release a ff asap and let us find this out for ourselves NOW!!!
01-06-2013, 09:06 PM   #7
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Haha. We don't need to wait for that. There are colleagues here who have 5Ds they have mount converters for and have been shooting with their FA 77s with...
They can tell us.
01-06-2013, 09:10 PM   #8
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Really ? Let's see some results then. If they look great I'll buy a used Canon 5 D and a converter.
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01-06-2013, 09:14 PM   #9
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I don't have one myself - I've seen forum members here post to say they've gotten some great results with the FA77 on a 5D, but remember that AF function is lost with such a combination.
01-06-2013, 09:47 PM   #10
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This is based on some of my comments from the other thread:

The reason the FOV changes, all other variables kept the same is because the sensor size or film area changes. I specifically don't get into the equivalency with f values and depth of field because it creates way too much confusion. An f2.8 lens always acts like an f2.8 lens regardless if it is a m4/3 sensor, aps-c sensor (Canon or Pentax/Nikon), full frame even thought the angle of view changes. This is because neither the physical focal length nor the physical aperture change.

The FOV changes which is why the depth of filed changes when the same lens is used on different formats. For example, an FA 77/1.8 at f1.8 lets the same amount of light to the aps-c sensor at ISO 100 as it does to a roll of ISO 135 film. The actual difference is the sensor/film size, not the focal length or aperture.
01-06-2013, 09:48 PM   #11
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QuoteOriginally posted by Franky2step Quote
I suggest Pentax/Ricoh release a ff asap and let us find this out for ourselves NOW!!!
I currently use a F28/2.8, FA 50/1.4 and FA 77/1.8 LTD on an MZ-3 film body and various aps-c bodies (K200d, K20d, K-5, K-01 and *istD).
01-06-2013, 10:41 PM   #12
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QuoteOriginally posted by Blue Quote
The FOV changes which is why the depth of filed changes when the same lens is used on different formats.
If the shot is taken from the same position, the image is exactly the same as the full-frame one, only cropped, so the DoF is the same as well.
01-06-2013, 10:44 PM   #13
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QuoteOriginally posted by Blue Quote
For example, an FA 77/1.8 at f1.8 lets the same amount of light to the aps-c sensor at ISO 100 as it does to a roll of ISO 135 film. The actual difference is the sensor/film size, not the focal length or aperture.
This is wrong, as the APS-C sensor is smaller, less than half the light is captured by this sensor compared to 135 film or FF sensor. Light per area unit will be the same for both, but the sensor/film with larger area will collect more light.

But if using a converter to make a FF lens to APS-C lens, so the APS-C sensor can collect all the light from the lens. Then light per area unit will be higher on the APS-C sensor. Shutter speed will then be faster on the APS-C camera if using the same aperture and ISO as on a FF camera.
01-06-2013, 10:52 PM   #14
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QuoteOriginally posted by Ikarus Quote
When the shot is taken from the same position, the image is exactly the same as the full-frame one, only cropped, so the DoF is the same as well
When enlarged to the same size, however, there would be an apparent decrease in DoF - but this is just a magnification effect. As such, any benefit from FF must be with proportionally increased resolution from that of the smaller format sensor.
01-06-2013, 11:01 PM   #15
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QuoteOriginally posted by Fogel70 Quote
Light per area unit will be the same for both

This is what I believe Blue meant.

QuoteOriginally posted by Fogel70 Quote
But if using a converter to make a FF lens to APS-C lens, so the APS-C sensor can collect all the light from the lens. Then light per area unit will be higher on the APS-C sensor. Shutter speed will then be faster on the APS-C camera if using the same aperture and ISO as on a FF camera.
The reverse teleconverter would make the lens faster by virtue of increased light intensity rather than larger aperture. But that is essentially the same as just having the lens itself on the larger format camera, only with more resolution.
I don't know of the practical utility of such a reverse teleconverter. The degradation in IQ would negate any apparent benefit. If there were great advantages for such a gizmo, they would surely have been made available by now.
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