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10-29-2012, 09:14 AM   #1
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What does the F value mean for a lens

Well, this is the beginner section so here goes...

When you see the F value for a lens advertised (such as below for the DA 55-300) is this just saying the optimal F values to use on your camera?

Pentax SMC P DA Zoom lens - 55 mm - 300 mm - F/4.0-5.8

For example, I have the above lens but in my camera on Av mode I can go all the way to F values of 20 and higher. However, is the F/4.0-5.8 really saying that I should stick within these values? What is the impact of going outside of them?

10-29-2012, 09:20 AM - 1 Like   #2
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F designates a measure of the maximum amount of light that the lens allows in. The smaller the F value, the more light the lens will be able to let in. In the example above it means that if you were set the zoom lens at 55mm then you will not be able to go lower than F4 and if you zoom in to 300mm you would have further limit the light entering the lens bcs you would have to be at F5.8. Zooms tend to have larger F values as, while primes allow for smaller F values (more light can enter).

As you go into higher F values less light enters, so you have to compensate by lowering shutter speed (may need tripod). High F-values produce bigger depth of field (more stuff in the picture will be 'in focus' and sharp). This is good for shooting scenes like landscapes.


I hope this helps!
10-29-2012, 09:23 AM   #3
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Thanks oxidized, that makes perfect sense now.
10-29-2012, 09:34 AM   #4
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Another thing to note: zoom lenses that have an aperture range like F4-F5.6 are generally consumer zooms, while zoom lenses with a single maximum aperture like F4 or F2.8 are generally prosumer or professional lenses, and deliver better image quality.


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10-29-2012, 10:11 AM   #5
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QuoteOriginally posted by oxidized Quote
F designates a measure of the maximum amount of light that the lens allows in.
To be correct, that is actually the T value and not the F value but these numbers are connected though
The T value also takes into account the lens elements and the coatings so it's the true number of light that let the lens trough, where is the F value is simply a ratio between the entrance pupil and the focal length.

So for example F/4 prime will most likely let more light through then a F/4 zoom lens because the prime has less elements and more efficient formula/design.

I'm nitpicking here though.
10-29-2012, 10:32 AM - 1 Like   #6
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If you want the full explanation of the number and why it's an unusual scale, it's here. That's interesting but more than you really need. The basic idea is that like Oxidized said, it measures the light allowed into the camera. The light is controlled by the aperture on the lens, which opens or closes. Because lenses can vary a lot in construction, focal length, etc., the f number takes all those variables and turns them into one.

When it comes down to actual photos, smaller f numbers mean more light and less depth of field (part of the image that's in sharp focus). Here's a quick example with a 55mm lens at f1.4, about 20 inches away:



The photographer often has to balance letting in more light against not enough depth of field, choosing an aperture that works the best for each shot.

Another consideration is image quality, which varies as you change the aperture. This is from one of my lens tests. The camera is on a tripod, focused carefully on newspapers taped to a wall. The image is a series of crops from one corner of each shot, pasted together. The only thing that changes in each shot is the lens aperture. In this case, the left side is at f2.8, then I stop down one full stop (f4, f5.6, f8, f11).



As you can see, the image gets sharper at smaller apertures. This is not depth of field, it's just a property of all lenses that they get better at smaller apertures. Other aspects of quality get better too. Contrast and colors get better. This lens has some light loss in the corners at the widest apertures, so you can see how the corners get brighter with smaller apertures. If I had continued taking shots at even smaller apertures (f16, f22, f27, f32) another factor (diffraction) would have made the images softer.

Don't get completely overwhelmed by the f-number, especially the effect on quality. If your shot has the right focus, depth of field and no camera shake, no one will notice the slight difference another aperture might have made. Everyone will notice when focus is wrong.
10-29-2012, 11:39 AM   #7
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Cool, thanks guys. That pic of the dog is nice.

The pictures of the newspapers is interesting and counterintuitive to me. Was that with the shutter speeds the same in each picture? I would have thought that a larger aperture would be sharper as it lets more light in so it's interesting to see it is the opposite. Or is it just the edges/corners that are less detailed and the centre is better with larger aperture? I shall try this out with my lenses, seems like a good way to learn a bit about them.
10-29-2012, 11:58 AM   #8
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QuoteOriginally posted by thechumpen Quote
Cool, thanks guys. That pic of the dog is nice.

The pictures of the newspapers is interesting and counterintuitive to me. Was that with the shutter speeds the same in each picture? I would have thought that a larger aperture would be sharper as it lets more light in so it's interesting to see it is the opposite. Or is it just the edges/corners that are less detailed and the centre is better with larger aperture? I shall try this out with my lenses, seems like a good way to learn a bit about them.
Depth of field - Wikipedia, the free encyclopedia

10-29-2012, 12:35 PM   #9
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Anvh really has the correct answer.

In simple terms the F number is the ratio between focal length and diameter.

the reason a lot of zooms are F4-5.6 etc is that the diameter of the lens iris is easy to keep a fixed value, and therefore as you zoom to longer focal lengths the ratio of focal length / diameter increases.
10-29-2012, 01:28 PM   #10
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It's a pricing thing. The smaller the number, the higher the price.
10-29-2012, 07:07 PM   #11
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QuoteQuote:
However, is the F/4.0-5.8 really saying that I should stick within these values?
[From original post]

To answer that question, no. The reasons have been explained pretty well but nobody has exactly answered that yet. In actuality most people tend to say stay around f8 if you can, for better sharpness. The number printed on the ring is the largest aperture that lens offers, not a specific recommendation. For me, f8 is usually my starting point. I go from there if necessary, or if I want to play with depth of field some.
10-29-2012, 08:41 PM   #12
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QuoteOriginally posted by thechumpen Quote
Cool, thanks guys. That pic of the dog is nice.

The pictures of the newspapers is interesting and counterintuitive to me. Was that with the shutter speeds the same in each picture? I would have thought that a larger aperture would be sharper as it lets more light in so it's interesting to see it is the opposite. Or is it just the edges/corners that are less detailed and the centre is better with larger aperture? I shall try this out with my lenses, seems like a good way to learn a bit about them.
Good catch, the shutter speed was faster for wide apertures, so I was wrong to say that everything was the same between shots. This test is a good use for the AE-L button. I can use Av mode, lock the exposure, then as I turn the e-dial to another aperture setting, the shutter speed automatically adjusts to keep the same image brightness.

The center does not change sharpness as radically as the corners do. The difference depends on the quality of the lens and what it was designed for.

A rough explanation of why lenses are not as sharp wide-open: the aperture controls stray light rays and sends them through the center of all the elements. That's about where my optics knowledge stops.
10-30-2012, 10:32 AM   #13
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QuoteOriginally posted by Lowell Goudge Quote

In simple terms the F number is the ratio between focal length and diameter.
We should actually call it the F ratio instead of number, maybe idea to start that as of today?
10-30-2012, 11:58 AM   #14
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This subject of focal ratios can get complicated, especially when you have a lens that doesn't fit the normal definition. In a reversed telephoto design (wide angle) one can have as an example, a 45mm wide lens (medium format) with an entrance pupil of 60mm. The focal ratio would appear to be f/.75. However, the lens is given an f/4 by Pentax. The reason for this slower designation is because of the design. Most of the front elements are negative and therefore cause the light rays to diverge. This causes much of the light to be lost into the lens baffling and not reach the film /sensor.
10-30-2012, 12:24 PM   #15
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QuoteOriginally posted by Just1MoreDave Quote
If you want the full explanation of the number and why it's an unusual scale, it's here. That's interesting but more than you really need. The basic idea is that like Oxidized said, it measures the light allowed into the camera. The light is controlled by the aperture on the lens, which opens or closes. Because lenses can vary a lot in construction, focal length, etc., the f number takes all those variables and turns them into one.

When it comes down to actual photos, smaller f numbers mean more light and less depth of field (part of the image that's in sharp focus). Here's a quick example with a 55mm lens at f1.4, about 20 inches away:



The photographer often has to balance letting in more light against not enough depth of field, choosing an aperture that works the best for each shot.

Another consideration is image quality, which varies as you change the aperture. This is from one of my lens tests. The camera is on a tripod, focused carefully on newspapers taped to a wall. The image is a series of crops from one corner of each shot, pasted together. The only thing that changes in each shot is the lens aperture. In this case, the left side is at f2.8, then I stop down one full stop (f4, f5.6, f8, f11).



As you can see, the image gets sharper at smaller apertures. This is not depth of field, it's just a property of all lenses that they get better at smaller apertures. Other aspects of quality get better too. Contrast and colors get better. This lens has some light loss in the corners at the widest apertures, so you can see how the corners get brighter with smaller apertures. If I had continued taking shots at even smaller apertures (f16, f22, f27, f32) another factor (diffraction) would have made the images softer.

Don't get completely overwhelmed by the f-number, especially the effect on quality. If your shot has the right focus, depth of field and no camera shake, no one will notice the slight difference another aperture might have made. Everyone will notice when focus is wrong.
What lens/camera was used for those newspaper crops? How far away was the lense from the newspaper? How big an area was cropped? Even at f2.8, a good lens should be a whole lot crisper than that at the edges.
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