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10-31-2014, 06:38 AM   #1
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Aperature Size accross lenses

I have a question. Last night I was setting up a small studio and was using my Pentax SMC-A 50MM 2.0 prime. Due to the lighting, I had to take my ISO down to 1600 in order to get even a decent picture with an Aperture of 2.0. then I tried my 18-135mm and at 3.5 aperture I only had to drop the ISO down to 400. I would have thought the same aperture regardless of lens would have provided the same amount of light, else how does the camera know what Aperture to set.


Last edited by kchamber4; 10-31-2014 at 06:39 AM. Reason: Didn't finish the question
10-31-2014, 06:47 AM   #2
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What about the shutter speed?
One thing I noticed with Pentax cameras is an odd behaviour with automatic adjustment of shutter speed with manual lenses. For some reason, the camera wants to stay 1/125 or faster with manual lenses. With other lenses, it goes down without a problem, and if you use a wide angle lens and SR, a slower shutter speed is no problem.
But if you switch to M mode and select the same shutter speed, ISO, and aperture, the exposure should be the same.
10-31-2014, 06:48 AM   #3
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Was the f 2.0 lens a manual focus lens? because the cameras can struggle with them Lowell has done charts on how they over and under expose at various F stops.
the other thiong is the metering is constrained on the manual; lenses they don't get the benefit of the full matrix metering. the real test would be expose both lenses using the same setting (ie full manual no auto. exposure). they should expose the same in that case this does however seem an extreem difference no matter how you look at it almost 4 stops of difference assuming the same shutter speed
10-31-2014, 07:05 AM   #4
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Thanks for the quick replies. Since the 50mm was an A lens, I was using TaV mode on my K-3. I couldn't even get to camera to stop flashing on the ISO until I got the speed down to 1/20th. I don't remember what the speed ended up on the 18-135mm but I believe it was around 1/80th.

If it clicked on the green button, the camera stopped the speed all the way down to 1/8th and a ISO of 400.

---------- Post added 10-31-14 at 09:09 AM ----------

Here is a link to some of the pictures I took last night with the Pentax SMC-A lens. (Just the ones with my wife and daughter's best friend. My daughter's pictures failed to load.
KCS Photography

10-31-2014, 07:38 AM   #5
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What focal length were you using with your DA 18-135? What the camera decides upon has a lot to do with what metering mode you are using and where the bright and dark areas are in the frame. If you were zoomed-in to a bright subject against a dark background with the DA 18-135, thus including less off the dark background, metering could adjust the exposure accordingly.

The DA 18-135 has thirteen elements vs. the A 50/2's five elements, so I doubt the DA 18-135 is passing more light at similar aperture setting, no matter how "enhanced" those elements may be.
11-01-2014, 06:39 AM   #6
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QuoteOriginally posted by AquaDome Quote
The DA 18-135 has thirteen elements vs. the A 50/2's five elements, so I doubt the DA 18-135 is passing more light at similar aperture setting, no matter how "enhanced" those elements may be.
+1, The 50 should transmit slightly more light at any given f stop due to losing less light internally. ( t stops on the 50 vs the 18-135 should favor the 50).

I like the idea of setting exposure manually and fixed and shooting the two lenses several times bracketed and the comparing them.
11-04-2014, 07:09 AM   #7
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QuoteOriginally posted by bradshea Quote
+1, The 50 should transmit slightly more light at any given f stop due to losing less light internally. ( t stops on the 50 vs the 18-135 should favor the 50).

I like the idea of setting exposure manually and fixed and shooting the two lenses several times bracketed and the comparing them.

Ideally f2.0 is f2.0 on any lens , but the light transmission varies as you point out. that is why we really should hav always used the cinematic t-stop which takes that into account so the exposure value is always the same at any given t stop.
light transmission can vary by as much as 50%
It wasn't an issue shooting stills, but most cameras now have a video component. to control exposure across different shots you need t stop so it flows and isn't jarring
11-04-2014, 10:26 AM   #8
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QuoteOriginally posted by eddie1960 Quote
Ideally f2.0 is f2.0 on any lens , but the light transmission varies as you point out. that is why we really should hav always used the cinematic t-stop which takes that into account so the exposure value is always the same at any given t stop.
light transmission can vary by as much as 50%
It wasn't an issue shooting stills, but most cameras now have a video component. to control exposure across different shots you need t stop so it flows and isn't jarring
With stills the only real value to a t-stop is when you have to compare the low light performance ahead of time for a purchase. Honestly with modern DSLR's the high ISO performance likely makes the small differences in T-stops less of an issue for stills as you point out but ultimately in a culture when people worry over 1/3 of a stop between lenses perhaps the t-stops are relevant.

11-04-2014, 10:56 AM   #9
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QuoteOriginally posted by bradshea Quote
With stills the only real value to a t-stop is when you have to compare the low light performance ahead of time for a purchase. Honestly with modern DSLR's the high ISO performance likely makes the small differences in T-stops less of an issue for stills as you point out but ultimately in a culture when people worry over 1/3 of a stop between lenses perhaps the t-stops are relevant.
more relevant because DSLRs and MILCs are also Video Cameras. for 99.9% of the users no big deal but for anyone who actually wants to use them for film making a very big deal. Aside from comparing lens performance and explaining the difference in cost between lenses it had zero use for Film Still shooters (which of course is the reason we just use F stop because it's good enough
11-04-2014, 11:05 AM   #10
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QuoteOriginally posted by Na Horuk Quote
What about the shutter speed?
One thing I noticed with Pentax cameras is an odd behaviour with automatic adjustment of shutter speed with manual lenses. For some reason, the camera wants to stay 1/125 or faster with manual lenses. With other lenses, it goes down without a problem, and if you use a wide angle lens and SR, a slower shutter speed is no problem.
But if you switch to M mode and select the same shutter speed, ISO, and aperture, the exposure should be the same.
It is normal behavior and quite reasonable in my mind. The camera has lens profiles for DA lenses and uses that for the program line while on lenses it doesn't recognize it will have a somewhat conservative behavior.
11-04-2014, 12:31 PM   #11
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QuoteOriginally posted by Na Horuk Quote
One thing I noticed with Pentax cameras is an odd behaviour with automatic adjustment of shutter speed with manual lenses. For some reason, the camera wants to stay 1/125 or faster with manual lenses.
I have not experienced this, but it does not surprise me. By manual lenses, I assume you are talking about manual focus lenses that have "A" contacts (i.e. Pentax-A). According to Pentax (personal communication), when an A-series lenses (no data contact) is mounted, the camera defaults to a different set of program lines than what is used with AF lenses. A few of the details may be found on this thread:

https://www.pentaxforums.com/forums/172-pentax-k-3/262923-k-3-pentax-lenses-program-line.html

This, apparently, is an undocumented "feature".


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11-04-2014, 12:35 PM   #12
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QuoteOriginally posted by stevebrot Quote
By manual lenses, I assume you are talking about manual focus lenses that have "A" contacts (i.e. Pentax-A).
Yes, I meant that! For example, the Samyang 14mm f2.8. It doesn't communicate focal length, so you have to enter it by hand. But the lens does have A contacts. Even though I input 12mm into the camera, it still chooses 1/125 as the lowest shutter speed.
11-04-2014, 05:57 PM   #13
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QuoteOriginally posted by Pablom Quote
It is normal behavior and quite reasonable in my mind. The camera has lens profiles for DA lenses and uses that for the program line while on lenses it doesn't recognize it will have a somewhat conservative behavior.
Is this still true outside of P mode? I didn't think the MTF was used in any other mode (like AV, TAV, etc.).
11-04-2014, 06:31 PM   #14
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QuoteOriginally posted by eddie1960 Quote
Ideally f2.0 is f2.0 on any lens , but the light transmission varies as you point out. that is why we really should hav always used the cinematic t-stop which takes that into account so the exposure value is always the same at any given t stop.
light transmission can vary by as much as 50%
T-Stops are now irrelevant?

As I read, -Long ago, Asahi Pentax was the first camera maker to introduce Super-Multi-Coating, specifically to increase light transmission by reducing reflection.
The claim was 0.998 transmission per surface with a 7 layer S-M-C coating (Ref Asahi Pentax "Complete System of Photography"

For a typical 7 element fixed focal Pentax lens, the relative transmission would be (0.998^14) = 0.972.
Compared with , 1 stop down, being 0.5 times original.

In his section on light transmission , Cox (in 1974) says " The advent of surface coating, as a means of reducing reflection losses, minimised the importance of T-Stops".
11-04-2014, 07:29 PM   #15
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What is f/stop?

Don't forget, f/stop is just an expression or ratio of a given aperture size relative to the focal length. And, there is concept referred to as "working f-stop" - basically that changing focus can have an impact on light transmission. As change focus, you change the amount of light that can get to your sensor. If you've ever looked closely as you change focus on a fast lens, you can see the image size change slightly (also referred to as "breathing" in film/video). If focusing makes the image zoom & out - even slightly -then you are changing the amount of photons hitting the sensor, and thus, the exposure.

The "f" is focal length and the number is the divisor. So, a 100mm lens at f/2.8 has an aperture size (a hole) that is 35.714mm in diameter. A 50mm lens at f/1.4 also has an aperture size of 35.714mm in diameter. On the surface, one might think that, given the same shutter speed and ISO, these 2 lenses at those settings (f/2.8 and f/1.4, respectively) would allow for the same amount of light to hit the sensor but we all know through practical experience that this is not the case! Why?

Well, the thing to remember about focal lengths is that when you double them, you are actually halving the total amount of light you are sending to the sensor or film plane. This is easier to visualize when you think about the whole relationship between what your lens is seeing (field of view) and it's relative aperture size. If you think about a very short focal length lens (an ultra wide wide lens) and a very long focal length lens (a telephoto) and how big of an area (again, the field of view) each gathers light from, it becomes very easy to visualize how much of a larger area the short focal length lens has to gather light from.

Picture this - you have a large box the size of building filled edge to edge with rows of lit candles. If you were to pick a lens - say 20mm - that could cover the whole box within it's field of view, it would gather the light from every one of the candles. If you were to pick another lens - say 300mm - and shoot the same scene from the same distance, it might only gather light from one of the candles. If both lenses in this example were theoretically capable of a setting of f/2 and that was the exposure you used, than the measurement of apertures would be 10mm in diameter for the ultra wide lens and 150mm in diameter for the telephoto (that's a BIG lens, lol!) You would think that the telephoto, having a hole 15x larger than the ultra wide would be 15 times brighter, but it won't be because it's taking an area of illuminance that is 15 times smaller than the wide lens, and thus gathering 15 times less light from the scene. So in the end, their exposures for f/2 should come in very close to each other when you consider the total amount of light gathered. But this can vary, too...

See, there are other minor variations in light transmission that still come into play with different lenses. The number of elements, the amount or type of coatings on the lens, the previously mentioned focus setting, etc. Each of these can have an effect on the amount of light that reaches the film plane and because an f/stop is just a mathematically expression of the relative aperture size for a given focal length that does not take into account those other factors, you can have 2 lenses of different focal lengths provide a different amount of light even if they are set to the same f/stop.

Also, when you consider the example above, what you're shooting can also have an impact on whether your exposure is sufficient or not. The 20mm lens covering the giant box of candles won't have much variance if you are shooting all the candles. The telephoto, on the other hand, might have all sorts of variance from the same distance. You might get a candle flame in the frame which gives you one exposure, or you might shoot the blackness between 2 candles for a completely different exposure. And what if you could reposition the telephoto, back it up far enough to get all the candles? Well, don't forget the other part of the equation when you are talking about light and distance is the scattering of photons. This is easier to see in practice if you've ever worked with macro-tubes. When you enlarge an image through distance (moving away to see a larger area or magnifying the projected the image from a lens), you lose the total number of photons that actual hit your sensor or film plane. You are taking the total number of photons for a given part of an image and distributing them over a larger area. This is why a 50mm f/1.4 gets darker and darker when you add macro tubes and why a fast telephoto lens needs to be so large.

That's where t stops come in. A "t' measurement of a lens is actually a measurement of the light transmitting properties of a lens. T stops are used on cinematic lenses because of the importance of having matching shots in film & television. Say you're shooting a scene in a movie with prime lenses and you shoot a medium wide shot with a 35mm lens at f/2.8 and you want to switch lenses to an 85mm for a close up. If your lenses only have f/stop clicks and you put that 85mm on you might be surprised to find that suddenly you have to adjust your lighting for the scene when you set that 85mm to f/2.8. A 35mm at f/2.8 and an 85mm at f/2.8 could actually give you slightly different amounts of light to your focal plane and cause problems when you go to edit your scene together. But a 35mm t/2.8 and an 85mm t/2.8 will give you EXACTLY the same amount of light and you won't have to change anything about the lighting because the lens maker designed the lenses to have matching t stops.

Hope that explains it for you! Actually, I hope more that I got it right, lol...
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