[dkpentax]

Most zoom lenses accomplish their zoom functions by moving some lens elements around in relation to one another, and virtually all zoom lenses have varying performance across their zoom range.

The modern lens designs of using internal focus on prime lenses means they basically to the same thing. That implies that an internal focus prime lens is liable to have varying performance if you compare its infinity focus quality versus its minimum focus distance quality.

I have not noticed any such tests ever published, or any demonstration that this theory isn't correct.

[ramseybuckeye]

A non internal focus prime lens moves the front element to focus. Why would that be any different as a variable?

[dkpentax]

Most old lenses move the entire lens as one unit, not just the front element.

[Bob 256]

Internally focused lenses move a different lens set to accomplish the same as a moving the front element or the whole lens. There might be some small variations in the IQ over that range (and that holds true for a front element focused lens), but it would be slight. Anything that moves, changes the lens formula, and even moving the entire lens to focus closer than infinity affects it so a small degree. It might be hard to evaluate because you need to develop a lens target for each of those focus points (different magnification applies) and even repositioning the camera or target could alter results making a good test laborious and difficult to interpret.

[photoptimist]

All lenses (even the simplest primes) have different sharpness at different focus distances ... and at different parts of the frame ... and at different apertures ... and at different wavelengths.

The so-called "thin-lens" formula for how a lens focuses a point on some distant subject to a point on the sensor is only an approximation.

Designers try to create sufficient sharpness at a different distances, different places on the frame, and different wavelengths by adding more elements, groups, aspherical shapes, exotic glass, and both internal and external moving parts in a lens. The more complex the lens, the better the opportunity to create uniform performance.

But an added design constraint such as "internal focus" or "no focus breathing" or compact length or small front filter or nice bokeh or .... can make it harder for the designer to get sharp focus at all focus distances, all points of the frame, and all wavelengths.

The net effect is that a modern 15-element, $1,500 internal focus prime will likely have less uniform sharpness than a modern 15-element, $1,500 external focus prime but that internal focus prime may have better uniformity than any cheaper or simpler prime.

Finally, It's really hard to make accurate generalizations about photographic lenses because they vary on some many different dimensions of cost, complexity, build quality, and design goals.

[ramseybuckeye]

Most old lenses move the entire lens as one unit, not just the front element.

But still a movement in distance from the sensor.

[photoptimist]

P.S. Emmanuel BIGLER wrote a really good response (see How are lenses optimized for a particular distance or ratio ? [Archive] - Large Format Photography Forum) to a question about lenses being optimized for a particular distance.

A summary of his post basically says:

1) A symmetric optical formula creates nearly a perfect lens for 1:1 macro (but isn't very sharp at ∞). That pushes designers toward asymmetric optical formulae.

2) Most large format lenses are optimized for something like 1:10 or 1:20 which is for subjects that are 11X or 21X the focal length from the film plane. That ratio biases performance toward sharpness at infinity but may not perform well close-up.

3) Floating element designs can broaden the range of subject distances that offer good sharpness (Some LF lenses even have a removable of internal shim for switching between 1:1 optimization and long-distance optimization).

4) Lenses for 35mm photography often use very asymmetric lens formulas because: 1) Wide angle lenses must use a retrofocus design due to the long distance between the lens center and sensor; 2) telephoto lenses use an asymmetric design to make the lens much shorter than the focal length.

[Lowell Goudge]

A non internal focus prime lens moves the front element to focus. Why would that be any different as a variable?

Traditional MF primes moved the whole group this changed with the K28/2 with the fixed rear element. Internal focus lenses change focal length to focus, as opposed to moving the lenses.

The OP has an interesting point.

[Digitalis]

That implies that an internal focus prime lens is liable to have varying performance if you compare its infinity focus quality versus its minimum focus distance quality.

Primes based upon IF designs certainly have issues, but the compromises involved are nothing compared to what are encountered with zoom lens designs. @photopimist stated above: the use of IF with floating elements in primes allows for a great deal of flexibility in optimization rather than presenting a need for more overt correction for compromising optical errors. A classic example of this is the Pentax FA*200mm f/4 IF ED MACRO and Sigma 180mm f/3.5 APO EX Macro I work with - both have internalized focusing and both are extremely well optimized macro lenses. The biggest drawback of IF designs is the loss of effective focal length at closer focusing distances, (which can be quite severe depending upon the focal lengths and focus distances involved) and light loss*. In cases of retro-focus wide angles floating elements and IF are used to create compacter lenses with better correction than was was possible before - the use of floating elements in particular allows designers to optimize bokeh and aberration control at key focus distances**, which most manufacturers seek to optimize performance around MFD and ∞. The SMCP-FA31mm f/1.8 limited and the more recent DFA21mm f/2.4 ASPH Limited benefits from these advanced design techniques, along with exotic glass types and aspherics which are used to control aberrations. In the optical design interview transcripts on the recent DFA21mm f/2.4 Pentax effectively made an STF version of the lens*** - this prototype had the ability to shift groups of elements independent from one another to give a specific result.



*Vignetting can also be problematic,though the degree of light loss is substantially less than it would be if extension tubes were involved.
** The upshot of having a lens that does not extend in any way across the focus range is the reduced complexity needed for weather proofing.
*** if it ever sells like the SMCP-K 20mm f/1.4 prototype did, I'd expect the price to be astronomically high.

[dkpentax]

Traditional MF primes moved the whole group this changed with the K28/2 with the fixed rear element. Internal focus lenses change focal length to focus, as opposed to moving the lenses.

The OP has an interesting point.

Do you mean the 28mm Hollywood lens has a fixed rear element but the rest of the elements move? I have one of these and all the elements certainly move as one group when it is focused. Did you mean some other 28mm lens?

[Digitalis]

Do you mean the 28mm Hollywood lens has a fixed rear element but the rest of the elements move? I have one of these and all the elements certainly move as one group when it is focused. Did you mean some other 28mm lens?

The Bojidar Dimitrov K mount lens database distinctly mentions floating elements as a feature of the K28mm f/2's optical design.



I will also mention the SMCP-FA77mm f/1.8 Has in Pentaxs' own unique nomenclature dubs: FREE - Fixed Rear Element Extension. The rear element which is fixed relative to the focusing group.

[beholder3]

I want to mention that there are macro „primes“ which focus by zooming = adjusting focal length. So while they are advertised as fixed focal length macro they actually are zooms. Fun, isnt it?

Kinda blurs the line between focusing and zooming in discussions.

But then we are in times where makers sell digital zoom and autocrop as wonderful features under the marketing slogan „breathing correction“. And the beginners coming from smartphones swallow it.

[pentaxus]

I think the 28mm f/2 has floating elements like my 50mm 2.8 A lens. All lens elements move during focus but some elements inside change spacing during focus.

[abruzzi]

I think the 28mm f/2 has floating elements like my 50mm 2.8 A lens. All lens elements move during focus but some elements inside change spacing during focus.

a few other examples--some Nikon wides had "CRC" or close range correction to correct optics when focused very close. Also the Mamiya RB67 focuses lenses by moving the enire lens on a bellows, so no focus ring, but some lenses had a "FLE" or floating lens elemnent. So you focused using the camera body, then read the distance off the scale and dialed that into the FLE ring.

[photoptimist]

I think the 28mm f/2 has floating elements like my 50mm 2.8 A lens. All lens elements move during focus but some elements inside change spacing during focus.

The Vivitar Series 1 28m f/1.9 is like this, too. The rear elements move away from the film plane as focus moves from infinity to about 1.3 feet but the front group stays still (reducing the distance between the back and front groups). Then between 1.3 feet and 0.9 feet the front element scoots out a short distance but not as much as the rear group continues to move.