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05-01-2009, 09:15 AM   #1
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constant f/4 (stupid question)

who and why would you need a constant F/4?

f/4 is rather slow so its not the speed... does the fact that its a constant aperture improve the quality of the image?

I asking BC these lenses are usually very expensive for a slow lens and even compare to some fast lenses

05-01-2009, 10:36 AM   #2
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Howdy neighbor! First of all it's not a stupid question. It's not that it's F4 it's that it's constant aperture. Generally speaking, constant aperture lenses tend to be higher quality lenses. More care has been taken in both design and manufacture. Take for example the Pentax F 70-210 F4-5.6 and the Pentax A 70-210 F4. If you check out our database here you will see the two lens rate about the same and cost about the same despite the fact that the F is an auto focus lens. That's because the IQ of the A is better than the IQ of the F (IMHO). The other advantage of a constant aperture lens is that there is a lot less refocusing as you zoom. I've gotten rid of most of my variable aperture lenses as I feel that I get better IQ from constant aperture lenses. The only variable lens I still have is the Sigma 17-70, and I use that lens only when traveling.
Don't forget that lens speed is only one aspect of total lens IQ, there is also color rendition, resistance to flare and CA, general sharpness both center and wide open, distortion, and bokeh. And probably others I've forgotten. I've found that constant aperture lenses generally handle the above better. I think that's because people who want a premium lens expect constant aperture. So when manufacturers make a 'top shelf' lens, constant aperture is one characteristic they include.

NaCl(it's not about the speed)H2O
05-01-2009, 10:47 AM   #3
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Also, so that you don't have to change your shutter speed whenever you zoom in our out.
05-01-2009, 10:51 AM   #4
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It's a bigger deal if you shoot in "M" mode mostly, as it means your exposure won't change as you zoom as it might if you were starting at f/4 and found yourself at f/5.6 before long. And even in Av mode, it means changing shutter speeds as you zoom, which might be disconcerting in some settings. But probably not a big deal on any count.

On the other hand, while f/4 isn't fast, it's better than f/5.6 or f/6.7, which is what the long end of otherwise comparable zooms might limit you to.

05-01-2009, 10:53 AM   #5
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I wouldn't call f/4 a particularly slow lens for a zoom. A lot of zoom lenses are much slower. For example, I have a Sigma 28-300 which is f/3.5 at 28mm and f/6.3 at 300mm. Now f/6.3 is slow. When zoomed in, the lens is unusable in dim light.

I also own the Pentax-A 70-210 f/4 that NaClH2O mentioned above. It a wonderfully sarp lens. And for me, the difference between the Pentax-A and (for example) a lens like the Sigma 70-200 f/2.8 is not just that the Sigma is one stop brighter, but that the Pentax was $550 cheaper.
05-01-2009, 11:03 AM   #6
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In addition to the reasons cited above, a constant aperture allows the use of auto flash systems (the ones where the sensor is in the flash). Auto flash systems operate so that you select an aperture on the camera and the flash, and the flash thinks it knows the aperture on your lens. For example the F 70-210 mentioned above would underexpose a full stop at the long end if you set the flash for f/4.
05-01-2009, 11:19 AM   #7
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Thanks guys!

I started photography less then a year ago most of this time it was winter so I shoot mostly indoor or in dark and cloudy weather, I think I can count on one hand the # of times I got above f/3.5

Now sumer time is a different story.
I find that I need longer lenses and control DOF better . I am saving my money for a new lens and I will buy next month

For now it's going to be either the 50-135 or the 77 but I am checking my options and pentax have a lot of constant F/4 lenses
05-01-2009, 11:57 AM   #8
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First i must apologise for not referring to original picture source since i couldn't find it any more. So if you know it, reference it. Correct me if im wrong in some statements as well.

But for an ideal lens the main quality limiting factor is diffraction. It is determined by lens aperture. The smaller it gets the worse are diffraction.
On a real lens the wider are aperture the more quality decrease you get from aberrations, vignetting, contrast and resolution loss. If those are equal at the maximum (lowest F num.) aperture then quality would increase equally when stopping down.
This means you'd reach the same quality faster with F4 than with some f5.6 lens and could maintain it with broader F range.
If F4 to F5.6 seems like a little difference, then counting this and the fact that usually F4 lens produces better picture already at F4 (so you'd have to stop down F5.6 lens even more) then something like F4@F5.6 Vs F5.6@F11 already makes a huge difference (like hand-held vs tripod). And you'd already started to go into noticeable diffraction with F5.6@F11 while F4@F5.6 still only gained image quality.

This chart also in some ways explain why you get better quality with larger medium. The falling quality is extended further to the right side with the same FOV lens.

Attached Images
 

Last edited by ytterbium; 05-01-2009 at 12:03 PM.
05-01-2009, 12:07 PM   #9
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QuoteOriginally posted by NaClH2O Quote
The other advantage of a constant aperture lens is that there is a lot less refocusing as you zoom.
I'm confused as what the lens aperture being constant or not has to do with focusing. Can you please elaborate on this.
05-01-2009, 12:16 PM   #10
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QuoteOriginally posted by ytterbium Quote
But for an ideal lens the main quality limiting factor is diffraction. It is determined by lens aperture. The smaller it gets the worse are diffraction.

nice thank you for that!!

so I guess according to this picture I should start shooting on f/8 more often
I am spoiled I have the 31mm this lens looks good no matter what F it is
05-01-2009, 12:18 PM   #11
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QuoteOriginally posted by zplus Quote
Also, so that you don't have to change your shutter speed whenever you zoom in our out.

Perhaps if you zoom in on a dark subject you may have to expose for longer time? Or if you zoom out and brighter objects are included you may need to expose for less time?
05-01-2009, 12:38 PM   #12
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QuoteOriginally posted by redpigeons Quote
nice thank you for that!!

so I guess according to this picture I should start shooting on f/8 more often
I am spoiled I have the 31mm this lens looks good no matter what F it is
Not necessary F8. Its just an approximate graph, but may actually be quite correct for 31/1.8.
In reality it may be much more complex and in general depends on focal length, pixel size and maximum aperture.
Since 31 is 1.8 it may reach its maximum quality already somewhere around F3.5..F4 and then start to decrease depending on pixel size somewhere around F8..F11.
So you could pick any aperture in F3.5 to F8 range that best suits your needs (DOF, speed) and still get the best possible quality.

Since aperture is calculated F=f/d then the physical aperture size is d=f/F e.g:
d=31/8=3,875 mm, for f=14mm lens you'd have to use F=14/3,875=F3.612 aperture to get the same diffraction limited resolution. Anything smaller (The same F8) would result in lower quality.
But since you get more DOF with wider angles you don't need to stop down that much. Unless the lens is still quite soapy at F3.5 (As something like sigma 10-20 are).


That's why you can get such spectacular landscapes on medium format. With the same FOV you can stop down much much more to completely eliminate any lens issues and not hit diffraction.
The fact that the medium is bigger means you can have lower unit resolution (lines/mm for e.g.) to have the same full resolution (lines/frame), so this pushes diffraction limit even further (as well as other resolution requirements).
Oops.. i'm afraid i'm repeating over discussed topics here.
05-01-2009, 07:42 PM   #13
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QuoteOriginally posted by kcmadr Quote
Perhaps if you zoom in on a dark subject you may have to expose for longer time? Or if you zoom out and brighter objects are included you may need to expose for less time?
If you look at zoom lenses that are not constant aperture, you will notice that as you zoom out your maximum aperture gets smaller. I have a 19-35mm lens that is a 3.5-4.5. That means at 19mm the maximum aperture is f3.5 and at 35mm the maximum aperture is f4.5. Given the same amount of brightness, the camera must use a slower shutter speed at f4.5 than at f3.5.

The actual physical size of the aperture may not have changed at all....but the focal length has. And f-stops are ratios of opening size divided into focal length. If you have a lens that is 100mm and f4, that means the opening is 25mm across. Now imagine it is a zoom that goes out to 200mm focal length...and the size of the opening is the same old 25mm. That means at 200mm it is f8, which is going to require an extra two stops exposure. So if shutter speed were 1/100 on the short end at 100/4 then it is going to have to be 1/25 on the long end at 200/8. You can see how that would be an annoyance.

Now imagine that the lens was designed so that the physical size of the aperture opening changed in relation to the zoom position so that no matter what focal length you had the lens zoomed to the aperture opening (when wide open) remained 1/4 of that size ("f4" in other words). One doesn't need to be a mechanical engineer to figure out that this is going to take more design work and result in a more complicated mechanism inside. And since this alone will push the price of the lens up, it makes no sense to put second rate glass in there. Hence the quality of constant aperture lenses tends to be better than that of variable aperture lenses.

As has been pointed out in an earlier reply, one of the benefits of a constant aperture zoom is if you are doing some flash shooting with a constant lighting output situation (not varied by technology such as wireless P-TTL). For example, I sometimes shoot events where I set up my remotely triggered lights and will have enough light for about an f4 exposure. If I used a variable aperture zoom I might be fine on the short end, as the maximum aperture there is greater than f4. I would just have the camera set af f4 and the lens would stop down that much and shoot. But on the long end of the zoom the maximum might be something like f5.6 and no matter what I do the lens isn't going to magically open up wider than that. I would be one stop underexposed. That could be corrected for by using a higher ISO and shooting f5.6, but that introduces a lot of hassle into things. With a constant aperture zoom I know I can work the lens at any position and be just fine. For example, my SMC-A 35-105/3.5 can shoot at f4 at 35mm or 105mm or anywhere in between. Same for my Tamron 28-75/2.8. It can shoot at f4 anywhere from 28mm to 75mm.
05-02-2009, 03:49 AM   #14
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QuoteOriginally posted by redpigeons Quote
who and why would you need a constant F/4?

f/4 is rather slow so its not the speed... does the fact that its a constant aperture improve the quality of the image?

I asking BC these lenses are usually very expensive for a slow lens and even compare to some fast lenses
Just to add something about manual mode: If you want to make a series of photos in one place, at different focal lengths, but exposed the same, you'd definitely want to shoot manual and with a constant aperture zoom--if you go the zoom route. This way, hypothetically speaking, you could take the shot of a stage and the singer, making sure the singer is exposed exactly the same in both photos.
05-03-2009, 12:16 AM   #15
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QuoteOriginally posted by SOldBear Quote
I'm confused as what the lens aperture being constant or not has to do with focusing. Can you please elaborate on this.
Constant aperture lenses are more likely to be zooms rather then vari-focal lenses.

Definition: Zoom, a lens that is able to change its focal length without changing its focal point.

Vari-focal, a lens that changes focal point when the focal length is adjusted.
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