[biz-engineer]

A few months ago, I posted about why Pentax program is too conservative , as it stops down lenses to f11, regardless of focal length. Based on FoV and relative position of camera to the ground, I defined a rule of thumb to set the proper aperture (aperture # = 2 x FL / 10). I used the rule of thumb for a few months, from the results I get I can say that it works, and for me it is defining how to select zoom lenses.

Based on the rule of thumb (f number setting = 2 x FL / 10), I find that the possible aperture settings of my DFA28-105 match well the rule of thumb for all in focus scene.

But I've always read the constant aperture lenses are better than variable aperture lenses, the reason why constant aperture lenses are better was never given.

For what I can see, lenses with variable apertures are smaller and cheaper, this observation doesn't match the saying that constant aperture lenses are better than variable aperture lenses.

So I'd tend to say that I don't want a Pentax DFA 70-200 f4, but I do want a smaller (yet optically sharp) DFA 70-200 f3.5-5.6 as a portable option to the DFA*70-200. This has consequences on how much weight I carry, and how much weight I'd carry.

Any thought that would bring knowledge about how to select a constant aperture lenses over a variable aperture zoom lens?

[beholder3]

Any thought that would bring knowledge about how to select a constant aperture lenses over a variable aperture zoom lens?

If you are aged 70+ and a changing aperture due to zooming is too challenging for you to mentally process, you will not like that.

If you are a dogmatic M mode user and have not learned to use the other modes properly, you will not like that.

I guess half of the thing is leftover from 30+ years old film shooting where cameras couldn't adjust on the fly while keeping a certain shutter speed. The rest is simply wanting a certain low F-number on the long end (which is the bit which makes it clunky and expensive). So actually it is less about "constant" it is more about "fast tele end". But that is a whole different question.

[biz-engineer]

If you are aged 70+ and a changing aperture due to zooming is too challenging for you to mentally process, you will not like that.If you are a dogmatic M mode user and have not learned to use the other modes properly, you will not like that.

I'm not sure about that, but I think you understood my point.

The rest is simply wanting a certain low F-number on the long end (which is the bit which makes it clunky and expensive

Yes, I agree, we seem to have the same understanding. The way I understand variable aperture zooms is that there is to glass area wasted because the min f stop diminished at the zoom extends, the design is optimized.


If I'm not wrong, I'd see wide aperture relevant for:
- wide angle zooms (with the DFA15-30 I can open up to f3.2 and still get all in focus frame at 15mm).
- portrait lenses for subject / background separation
- long tele lenses where having all sharp frame is anyway not possible without stopping down very much.

This leads me to believe that for my DFA24-70 , f2.8 is an overkill, not as good as primes for subject separation, yet quite bulky for landscape photography. The DFA70-200 f2.8 is a do it all portrait/sport lens, but it could easily be replaced by a 70-200 f3.5:5.6 and DFA*50 f1.4 , DFA85 1.4.

[Rondec]

A few months ago, I posted about why Pentax program is too conservative , as it stops down lenses to f11, regardless of focal length. Based on FoV and relative position of camera to the ground, I defined a rule of thumb to set the proper aperture (aperture # = 2 x FL / 10). I used the rule of thumb for a few months, from the results I get I can say that it works, and for me it is defining how to select zoom lenses.

Based on the rule of thumb (f number setting = 2 x FL / 10), I find that the possible aperture settings of my DFA28-105 match well the rule of thumb for all in focus scene.

But I've always read the constant aperture lenses are better than variable aperture lenses, the reason why constant aperture lenses are better was never given.

For what I can see, lenses with variable apertures are smaller and cheaper, this observation doesn't match the saying that constant aperture lenses are better than variable aperture lenses.

So I'd tend to say that I don't want a Pentax DFA 70-200 f4, but I do want a smaller (yet optically sharp) DFA 70-200 f3.5-5.6 as a portable option to the DFA*70-200. This has consequences on how much weight I carry, and how much weight I'd carry.

Any thought that would bring knowledge about how to select a constant aperture lenses over a variable aperture zoom lens?

I think traditionally constant aperture lenses are higher quality than variable aperture ones. They are more expensive and sharper wide open. I wouldn't expect a variable aperture 70-200, but probably a 70-300mm f4-5.6 is something like what we'll see. I imagine that it will show image quality along the lines of the 55-300 PLM for APS-C, which people seem to like quite a bit. I certainly will seriously consider adding such a lens to my line up for the reasons you say, it will be smaller and cheaper, than either the 70-200 f2.8 or f4 lenses.

The biggest problems with variable aperture lenses have to do with the long end. My DA 55-300 is f5.6 on the long end, but it really needs f8 to be sharp on the edges there and that needs quite a bit of light -- particularly if I am hand holding it.

[biz-engineer]

The biggest problems with variable aperture lenses have to do with the long end. My DA 55-300 is f5.6 on the long end, but it really needs f8 to be sharp on the edges there and that needs quite a bit of light -- particularly if I am hand holding it.

Agreed. If the lens is positioned horizontally, at human height (on a tripod or hand held) , the f stop required for an all in focus frame would be f32 at 300m, in which case the limitation is not the sharpness of the glass but the DoF. In practice, there won't be much foreground included in the frame at 300mm for a distant landscape scene (I think my rule of thumb will only work from wide angle to about 70mm), and the appropriate aperture will be something like f11, again two stops from wide open.

[microlight]

I’m not sure that constant aperture is always better quality than variable; my variable 20-40 beats my previous constant 16-45 at all points (except at having a constant aperture, obviously!), optically and in physical construction. Where there is a noticeable difference is in video, where with a variable aperture zoom, any attempt to zoom while recording is met with an audible jerky change of aperture as you zoom, whereas with a constant zoom, there is no such interruption.

[ffking]

I tend to assume that as fixed aperure zooms areitrinsically more expensive (as they need to be faster at the long end, therefore bigger glass) then the camera companies just go the whole hog - people who buy kit zooms only are already not going to buy them, and premium products in the range to aspire to is always a good thing. That and the hstorical aspects mentioned by beholder3 - but actually back in those days no zooms were any great shakes anyway.

Doesn'f fully explain why no premium variable aperture, but I suppose it's just market differentiation.

[kiwi_jono]

I suspect its mostly a perception thing, as in fixed aperture through the range is seen as more professional...

There is an advantage, however, in that whatever aperture you set in any in Av / Hyper programme / M, will remain the same - no matter how you change the zoom to adjust the frame. For example if you set it at f4, then as you zoom to the long end, its still f4 as you intended. Obviously thats not the case with a variable aperture lens.

Personally I don't mind variable aperture because in lenses like the DFA28-105, there is a benefit in weight, size and cost (but still its a great performer).

[Lowell Goudge]

Constant aperture lenses are most important when doing either manual flaps or auto flash, because when you zoom, exposure changes.

The issue with variable aperture is that the lens is fastest where you don’t need it, at short focal lengths, but when you consider hand held shots, you want the lens to be fast at the longest focal lengths. This means you pay more for the constant aperture lens.

Because you pay more, constantant aperture lenses are usually the higher end of a lens range, and usually give better image quality at maximum aperture.

I say usually because there will always be exceptions

[photoptimist]

There's no optical reason that constant aperture zooms are better. In fact, constant aperture would be another constraint on the designer that would force them to add complexity or other compromises. And some of those compromises are a bit of a hack. For example, designing a variable aperture zoom and then closing down the aperture slightly at the wide end -- the Pentax SMC Pentax-A 35-70mm F4 is really a f/2.8-4 lens with the wide-end stopped down.

That said, photographers' desire for constant aperture has created a self-reinforcing historical tendency for lens makers to distinguish their high-end versus low-end models with the constant-aperture versus variable-aperture feature. Constant-aperture zooms are only better because the lens maker has invested a lot more money and made a high-end lens.

[Turbotak]

...Constant-aperture zooms are only better because the lens maker has invested a lot more money and made a high-end lens.

This. Because of the added complexity, more investment is required, which means they might as well make it nearly as good as it can be made. It's like designing a magnificent engine, but then putting it on a cheap chassis. Might as well put the same effort into the entire car and sell it as a premium product.

[normhead]

How about an F-70-10
It's purple fine monster but if you can avoid high contrast situations.....
It's my "small portable but dense _I really don't want to carry a telephoto zoom but I might see something" option.

The first one taken on shopping trip. I just threw it into the camera bag, just in case, and low and behold, and opportunity presented itself.















My F- 70-210 slideshow.

It's not as hot now, but at the time it was pretty heavily corrected, 13 elements, and was a slightly ahead of the curve design.

It's the best $35 I ever spent on a lens.

[photoptimist]

How about an F-70-10
It's purple fine monster but if you can avoid high contrast situations.....
It's my "small portable but dense _I really don't want to carry a telephoto zoom but I might see something" option.

The first one taken on shopping trip. I just threw it into the camera bag, just in case, and low and behold, and opportunity presented itself.















My F- 70-210 slideshow.

It's not as hot now, but at the time it was pretty heavily corrected, 13 elements, and was a slightly ahead of the curve design.

It's the best $35 I ever spent on a lens.

Nice shots and and good evidence that a variable-aperture zoom can be a good constant companion!


Those who refuse to settle for less than the best often end up paying the most. So perhaps constant-aperture zooms are also known as "constant-profit zooms" inside the offices of lens makers. That said, I'm glad that some people are out there filling the coffers of the industry.

[John Poirier]

Many years ago constant aperture zooms were significantly more useful than variable aperture in certain situations. Before the days of TTL flash, it could be a pain to be constantly guestimating aperture settings to compensate for variable aperture zooming. The same is the case when using hand-held light meters. It was particularly inconvenient in fast-changing situations such as news photography. I think this is the main reason for fixed aperture zooms becoming the professional standard. As the professional standard, fixed aperture zooms have tended to be better built than variable aperture.

There are advantages to a 2.8 maximum aperture vs. the 3.5 or 4 common in variable aperture zooms for low-light work. This has become much less of an issue with modern sensors' high ISO performance.

I don't think fixed aperture lenses have any more potential for high optical quality than variable aperture. I have used some very good variable aperture zooms. From my perspective as a retired professional I see little value in the expense and size of fixed aperture zooms for most applications.

[MrB1]

There are advantages to a 2.8 maximum aperture vs. the 3.5 or 4 common in variable aperture zooms for low-light work. This has become much less of an issue with modern sensors' high ISO performance.

So, unless there is a need/desire for shorter depth-of-field, a lens such as the excellent, relatively compact and inexpensive D-FA 28-105 f/3.5-5.6 could be used as a constant aperture zoom at f/5.6, in conjunction with setting two stops extra ISO?

Philip