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09-18-2018, 01:33 PM   #1
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f stop conversion

I have a K3 ii aps-c and was evaluating the SMC FA 43mm prime lense. This being a full frame lense do I end up with about 2.8 f stop wide open dim lighting with this lense?

09-18-2018, 01:42 PM   #2
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No. You get F1.9 wide open.
09-18-2018, 01:48 PM - 2 Likes   #3
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No, Aperture is aperture. It doesn't change based on anything other than the amount of light the lens is letting through. The function of aperture is to provide a constant light intensity measurement on the film or sensor. It has nothing to do with what format is used.

It's when you are talking about Depth of Field that the effect of aperture changes. But the aperture itselff doesn't change.

Those people who tell you ƒ2 on APS-c is the same as ƒ2.8 on FF are seriously deluded.
An ƒ2.8, 100 ISO , 1/100s exposure on APSIC provides exactly the same exposure as ƒ2.8, 100 ISO, 1/100s on Full Frame, and those exposure settings will work the same on any camera ever made. Don't let anyone tell you different.

There are a lot of bloggers who generated a lot of income by creating click bait using that false information.
09-18-2018, 01:52 PM   #4
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QuoteOriginally posted by normhead Quote
No, Aperture is aperture. It doesn't change based on anything other than the amount of light the lens is letting through. The function of aperture is to provide a constant light intensity measurement on the film or sensor. It has nothing to do with what format is used.

It's when you are talking about Depth of Field that the effect of aperture changes. But the aperture itselff doesn't change.

Those people who tell you ƒ2 on APS-c is the same as ƒ2.8 on FF are seriously deluded.
An ƒ2.8, 100 ISO , 1/100s exposure on APSIC provides exactly the same exposure as ƒ2.8, 100 ISO, 1/100s on Full Frame, and those exposure settings will work on any camera ever made. Don't let anyone tell you different.
Exactly this!

This is one of those things you can explain 100 times to some people, and they seem to think otherwise. Aperture is nothing more than light on the film plane.

09-18-2018, 01:59 PM - 1 Like   #5
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You will get f/1.9 for exposure at the wide open aperture. Using the same lens, the main difference between 35mm full-frame and crop sensor is the field of view.
09-18-2018, 02:00 PM   #6
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What everyone has told you so far is correct, but let me add a little more colour.

If you use the 43mm f/1.9 lens on a full frame camera such as the K-1, you'll get the same field of view as you'd expect on film, and - obviously - a maximum aperture of f/1.9, which will result in a certain depth of field at any given distance.

If you put the same lens on an APS-C camera like your K-3II, it's still a 43mm lens with f/1.9 aperture. Nothing has changed. However... the APS-C sensor captures less of the projected image circle from the lens. It has a crop factor of 1.5x compared to a full frame sensor or 35mm film. As a result, you will get the same field of view and depth of field as a 64.5mm (43mm x 1.5) lens at f/2.8 (f/1.9 x 1.5) shot on a full frame camera at the same distance. But that's merely an illustration to describe the field of view and depth of field. In reality, nothing has changed with the lens. It's still a 43mm f/1.9 lens - but the field of view is cropped.
09-18-2018, 02:01 PM   #7
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Only slightly off-topic... but why aren't all lenses T-Stopped instead of F-Stopped?

An f/2.8 on one lens may produce a different exposure than a different f/2.8 lens (with ISO and Tv remaining the same).

Wouldn't it be better to report the T-Stops instead which would produce consistent results between lenses of of the same aperture?

09-18-2018, 02:06 PM - 3 Likes   #8
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QuoteOriginally posted by FozzFoster Quote
Wouldn't it be better to report the T-Stops instead which would produce consistent results between lenses of of the same aperture?
It wouldn't provide consistent results in terms of depth of field, though. Admittedly, the differences would be quite small, but I guess that's the choice - quote f-stop for accurate depth of field, or t-stop for accurate exposure. Neither measurement works for both simultaneously. To represent the whole story, lens manufacturers would need to quote both f-stop and t-stop.
09-18-2018, 02:08 PM   #9
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QuoteOriginally posted by BigMackCam Quote
quote f-stop for accurate depth of field, or t-stop for accurate exposure. Neither measurement works for both simultaneously.
Perfect! Thanks for the response BMC!
09-18-2018, 03:02 PM   #10
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QuoteOriginally posted by BigMackCam Quote
It wouldn't provide consistent results in terms of depth of field, though. Admittedly, the differences would be quite small, but I guess that's the choice - quote f-stop for accurate depth of field, or t-stop for accurate exposure. Neither measurement works for both simultaneously. To represent the whole story, lens manufacturers would need to quote both f-stop and t-stop.
There's something I never paid attention to before. F-stop for accurate DoF?

QuoteQuote:
A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens:

Since real lenses have transmittances of less than 100%, a lens's T-stop number is always greater than its f-number.[7]

With 8% loss per air-glass surface on lenses without coating, multicoating of lenses is the key in lens design to decrease transmittance losses of lenses. Some reviews of lenses do measure the t-stop or transmission rate in their benchmarks.[8][9] T-stops are sometimes used instead of f-numbers to more accurately determine exposure, particularly when using external light meters.[10] Lens transmittances of 60%–95% are typical.[11] T-stops are often used in cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable. Cinema camera lenses are typically calibrated in T-stops instead of f-numbers.[10] In still photography, without the need for rigorous consistency of all lenses and cameras used, slight differences in exposure are less important; however, T-stops are still used in some kinds of special-purpose lenses such as Smooth Trans Focus lenses by Minolta and Sony.

f-number - Wikipedia
I guess in practical sense, the difference between T-stop and F-stop is compensated for with the EV control. Although I almost always shoot a -.07 EV or less than the Aperute suggests, a lens's T-stop number is always greater than its f-number which would suggest I'm under-exposing full stop, on most of my lenses comparing to T-stop values. I'm not really sure the T-stop is of any practical value to most of us. F-stops which conveys the angle of the cone would seem to be more valuable. Although, I'm not convinced a lens with the element far up in the lens case will have the same cone value as one that extends back into the camera body. But all this while interesting falls in the category of "who really cares". The EV correction dial and meter behind the lens, with light measured leaving the lens, not entering it, makes T-stop irrelevant., and there are other factors to exposure more important than what the absolute transmittance of the lens is or the exact amount of light. Exposure is calculated by measuring what come out the back of the lens (TTL). Neither what the t-stop is or f-stop even matters to the exposure meter behind the lens. The meter doesn't care what the T-stop or F-stop is, it just measure light intensity. As stated it's more of relevance to people using hand held light meters, that can only measure what's in front of the sensor, not what's coming out the back of it.

T-stop is relevant only to systems without through the lens metering. Pentax made it unnecessary for SLRs and DSLRs almost 50 years ago.It only took other manufacturers a few years to catch up.

Of course Nikon and Canon users claimed it was a totally unnecessary conceit, and definitely not worth buying a camera for, until their brands came up with something.
Then it was the greatest thing since sliced bread.

Some things never change.

Last edited by normhead; 09-18-2018 at 03:21 PM.
09-18-2018, 03:12 PM   #11
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I'd quickly add - Given that the F stop is a measurement of area, and the T stop is that but adjusted according to the transmisivity of the glass (how much light hits the sensor). It is much easier to measure the F stop as all you really need is a ruler and primary school maths. To measure a T stop you need the ruler, slightly more advanced maths as well as accurate and well calibrated light measuring equipment.

That's pretty easy now, but go back 50 or 100 years and it would have been rather more difficult to measure.
09-18-2018, 03:17 PM   #12
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QuoteOriginally posted by normhead Quote
There's something I never paid attention to before. F-stop for accurate DoF?
No, me neither, Norm Nor have I considered the tiny differences in light from the minor t-stop differences. They're both equally relevant or irrelevant depending on our use cases, I guess.

We have a notion of what 50mm @ f/2 will provide in terms of depth of field at any given distance, and that has become a standard we work with. We kind of expect one 50mm f/2 lens to have the same depth of field as another 50mm f/2 lens. Of course, it's rarely exactly the same, I'm sure, because of minor differences in focal length and aperture size. There may be (probably is) even more to it that I'm not aware of. Still, assuming identical focal length and aperture size between two lenses, the depth of field should be the same, right? And it's a whole lot easier for stills photography folks to compare various lenses labelled "50mm f1:2" than "50mm T/2.2" vs "50mm T/2.1" vs "50mm T/2.4" (or however they're labelled)...

Last edited by BigMackCam; 09-18-2018 at 03:30 PM.
09-18-2018, 03:34 PM - 1 Like   #13
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The main reason for T stops is to try to keep lighting consistent in video between lens changes when using FILM. Photography usually has less of an issue with this with time to adjust each image to more gradually transition within a series. Video is much less forgiving of this but digital video is a little more forgiving. F Stop is completely sufficient with TTL metering. With external metering the T - stop would give slightly more accurate results but the difference is not terribly important in the digital era.
09-18-2018, 03:51 PM   #14
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having tested severial lenses the f-stop value was always korrekt was is written on the lens
what has darkend the lens was if you move the last lens away from your sensor what quickly resulted into less light
measuring this with a adapter or a macro ring gave me interesting results
only a small distance was already from f8 to f13
that was for a mirrorless camera tho

however having a d-lsr measure this didnt happen so quickly it gone down a bit but it took a while

cant the light be bend so the bokeh is the same as for F4 while having the f-stop of F2 ? according the light waves can have a positiv resonance
09-19-2018, 04:43 AM   #15
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I taught my partner to take photos without auto-everything mode. She asked me to, I'm not on some evangelical quest! She couldn't understand that F-stop is F-stop, regardless.
It took one loooooooong session with old manual lenses, auto lenses, moving the diaphragm lever on the lens with my thumb and changing lenses without changing camera settings for her to "get it". It's just a number, but a very important one.
To me, it's one of those "penny drop" items, once you've grasped the principle it's very simple, until that point, not so much. I think my explanation was lacking, not her ability to learn. I had no-one to teach me, save library books, a Praktica film camera and dirt cheap postal film development to allow me to experiment. I've included a couple of old negative scans from the MTL5 and Pentax 50mm - these show me working out what shutters and F-stops did. There were many worse attempts!
Once you understand how shutter speed and F-stop work together, then you can understand how ISO also fits in, and about DOF and hyper-focals. At that point, you can operate any camera, film or digital, and your concentration is left to the important thing - finding the picture and taking it.
We may not shoot so much onto film and plates any more, but the fundamentals still apply.
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