Primes vs Zooms

I am wondering what the visual and picture quality differences are between prime lenses and zoom lenses that would cause a DP to choose one over the other. Let me be more specific. Say you have a brand new full set of Zeiss prime lenses, and a brand new 10:1 Zeiss zoom lens and your shooting super 16. The lighting gives you a t-stop of 4.

Will there be any visual difference between your 25mm prime lens, and the zoom lens zoomed in to 25mm? Any difference between any of the primes at there focal lengths, and the zoom lens zoomed into to each respective focal length?

From what I heard or read elsewhere, in theory the primes are suppose to be sharper in general than the zoom. How much this is a factor with today's new Zeiss glass between the two types of lenses, I do not know. Also, I know that there is some breathing in the zoom when racking focus, which you do not get with the primes. I understand that a D.P. may choose a zoom lens over a set of primes for speed reasons when shooting, as dealing with changing lenses can be time consuming. But my core concern is visual difference.

Do you get less depth of field with prime lenses than you do with zooms, allowing you to blur out the background easier? Will the background be more in focus at t-4 at 50mm on a zoom than a prime at the same settings?

I know that shooting a test between the two would give me the answers, but any thoughts anyone would have at this point would be most helpful. Thanks!

Carlo Besasie
Gaffer - Milwaukee, WI

Essentially a zoom lens is asked to do a lot more tricks than a prime lens, so it can be a bit weaker in certain aspects they share. The zoom may not be quite as sharp, not have as good contrast or color reproduction. One of the biggest issues can be distortions. Your example of a Zeiss 10:1 is perfect. Zoom that lens all the way out to 10mm and it will show a good deal of barrel distortion or "pin-cushioning," where it looks like someone took a cube and blew air inside until it puffed out. Now zoom in to 100mm and see how the contrast gets a little mushy. Compare either ends of this lens to a prime of the same length and in general those same distortions will be much less.

Mitch Gross
NYC DP

Thanks Mitch for the info and tips on the Primes VS Zoom post.

Could I ask one more question pertaining to that issue? Could you, and/or any one else comment on the differences of depth of field between the two types of lenses?

Take the same scenario I presented before : You have a brand new full set of Zeiss prime lenses, and a brand new 10:1 Zeiss zoom lens and your shooting super 16. The lighting gives you a t-stop of 4.

Will there be any visual difference between your 25mm prime lens, and the zoom lens zoomed in to 25mm with regards to depth of field? Will the prime lens blur out the background more than the zoom lens at the same focal length and t-stop, or are they exactly the same. And, is the fact that you can focus much closer to the lens with a prime than a zoom a huge factor in the decision between using one or the other?

Thanks in advance for any thoughts you may have.

Cheers,
Carlo Besasie
Gaffer - Milwaukee, WI

As to depth of field, this is a mathematical formula based on physics, so zoom or prime should not matter. That said, the fact is that since primes are generally sharper lenses with fewer elements, the relative falloff of out of focus distances is generally more pronounced. This means that you can see the difference in sharpness more evidently on primes and therefore the effective depth of field is tighter on primes then it is on zooms.

As for close focusing issues, some zooms will let you get quite close, but this is definitely one of the many factors people take into account when favouring primes. The Cooke 18-100 zoom will focus to something like 18 inches, which is only a few inches in front of the lens housing. That lens is more than a foot long with a rather huge front element. Imagine being an actor trying to concentrate on a scene while that monster parked itself next to you.

Mitch Gross
NYC DP