The effect of lens 'size' on image quality?

S

Stab

Well-known member
When I see 'professional' camera setups with like Alexa's or the big Sony camera's, they usually have these huge (prime) lenses on it.

My question is actually, why are these expensive, professional lenses so big? Does it make them sharper? Does a big front element let actually in more light? Does it change the bokeh or the way it handles colors?

What makes them better than for instance the Zeiss compact Primes, or even smaller / cheaper cine primes?
Just wondering. Thanks.
 
Sets are often made to the same OD for easy alignment of the Follow Focus.

As for quality I guess going 'oversize' helps.. for example most lenses perform well a stop down from the 'open' - so make a 1.4 lens, call it 2.0 and it will actually be good at 2 - in general the bigness means you dont need to use the edge of the elements, light will hit at less obuse angles etc - all of this will aid performance?

S
 
Stab said:
When I see 'professional' camera setups with like Alexa's or the big Sony camera's, they usually have these huge (prime) lenses on it.

My question is actually, why are these expensive, professional lenses so big? Does it make them sharper? Does a big front element let actually in more light? Does it change the bokeh or the way it handles colors?

What makes them better than for instance the Zeiss compact Primes, or even smaller / cheaper cine primes?
Just wondering. Thanks.
Click to expand...


It's for many reasons, but even cinema lenses change drastically in size.

1) Internal telescoping. Now not all cinema lenses do this, but on photography lenses and older cinema lenses, when you focus from near to far, you'll often notice the length of the lens (the actual length of the barrel) grow or shrink. If the lens telescopes, it will start to mess press against your MB or filter. I have lots of issues with Mk1, mk2 SuperSpeeds because of this! Same with Panavision Super/Ultra speeds. If you don't set the MB perfectly in the middle of the telescoping range, you'll either meet resistance when the lens front presses firmly against the MB/filter, or the lens will "pull back" from the MB back ring and allow light through the back of the MB, possibly creating reflections in the image (if you are using filtration)! This can get more annoying if using a rubber donut and not a MB back ring.

2) Cinema lenses also like their lenses to stay relatively the same size. It's typically an impossibility to be perfect, but the variances are much less from lens to lens than with photography lenses. If you actually look at cinema lens designs, you can see that they do vary greatly. For instance, the from element of my 75mm lens is about the size of a quarter, while the front element of my 25mm is probably 6x the size. However, both lenses are identical in front diameter, iris ring placement, focus ring placement, and almost identical in weight and length. The 75mm, in actuality is a very tiny lens, but it's in a larger housing for all the reasons I'm typing now, but also for all the lenses to match. Matching lenses to be identical (or close to) in weight, size, length, and ring placement saves time and money on set.

a) if the focus/iris rings are the exact same placement on each lens, you do not have to move or readjust motors or follow focus placement.... ever.
b) if lenses are identical or close in length, you don't have to adjust the placement of the MB on the support rods.
c) if the lenses are identical in front diameter, you don't have to switch the donut or MB back.
d) if lenses are similar or identical in weight (and length), you need to re-balance the camera less often.

3) Greater size = greater girth. Greater girth =greater rotation. Moving 45 degree rotation on something small is less distance traveled than rotating 45 degrees on something larger. Many larger housing offer a longer throw of focus rotation thus slowing down the progression through the scale and also allowing more witness marks to be put on the lens. Also, many high end cinema lenses use Cam-followers instead of threaded focus systems to focus. These systems can really appreciate having more room in the lens housings and allow for the focus scale to be more mapped out, and not stuck with the quickly unfavorable typical escalation toward infinity.

4) Yes, it is my understanding that there are optical advantages to larger optical elements, however I don't think it's as much as you think. Lens coatings go a long way in many categories of making lenses kick ass, and I'm not an optical engineer/designer so I can't weigh in technically. I would think for somethings, like wide angle lenses, there are sure advantages, perhaps even for speed.


For the most part, I think many mid-range focal length professional lenses *could* be much smaller, than they are, but probably not as small as some photography lenses, as they need to hit a certain level of quality. For the reasons listed above, cinema style lenses benefit from being housed in larger, smoother functioning housings and that is the main reason for their size. The mechanics.
 
Damnit Ryan, that's what I would call an answer. Great read, thanks for sharing your knowledge. It all makes sense.
 
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