Difference between lenses of different focal lengths?

[Harrison1117]

If I have a prime lens with 50mm focal length and 1.8 max aperture, and a lens with 85mm focal length and 1.8 max aperture, what differences are there besides the zoom provided by the differing focal lengths? I know that 85mm, 1.8f is preferred for portraits for some reason. Is there some other special effect one lens might have that the other does not?

 

[Solarflare]

85mm is a better portrait lens than 50mm because you can keep more distance and the portrait will thus be less distorted.

Also you get a more shallow depth of field, allowing to make the background more blurry.

 

[Dao]

Different lenses usually construct differently and have different optical properties. It is all depends on which lenses you are talking about. The lens coatings, number of aperture blades, AF speed, focus mechanism. My EF 85mm f/1.8 has CA issue when shoot wide open while my EF 50mm f/1.4 does not. My EF 85mm f/1.8 and the EF 50 f/1.4 have 8 aperture blades while the f/1.8 only have 5. The out of focus blur (i.e. light) is rounder with more blades or when more spikes in the star light with more aperture blades when shooting a street light at night. (need to stop down the lens)

Other than that, it is mainly the DoF and the FoV difference. Of course, for the same framing, there will be a perspective distortion difference if you shoot with 85mm and 50mm due to subject to camera distance difference.

 

[Solarflare]

We specifically talk about differences in focal length though - and differences in focal length primarily means different viewing angles.

In that respect, the 85mm picture is in the 50mm picture, just its the smaller part in the middle of it.

This part in the center isnt exactly what a 85mm will give you, because the real 85mm will also give you more shallow depth of field, and of course the central part will give the same resolution. But from what you will get into the frame, the viewing angle, will be the same.


There are btw nice pictures in the net that demonstrate the effects of the various focal lengths in a portrait, for example this one:

http://petapixel.com/assets/uploads/2011/11/focallength_mini.jpg

It should be noted that the 19mm picture would have to be taken very close to the face of the person, and the distortions are abhorrend. While the 350mm picture could be taken at a distance of about 5 meters, which is considered ideal for beauty portraits.

Basically for beauty portraits you ideally want a distance of 5 meters to your subject. That means 85mm will give you about full body and 300mm will give you about head with shoulders, always of course depending upon the individual person.

Of course 85mm will still distort much less if you take a head shot, than a 19mm.

 

[KmH]

Also you get a more shallow depth of field, allowing to make the background more blurry.

That is not true if subject scale in the image frame is kept constant between focal lengths.
However, the background will be somewhat magnified, making it seem to be blurred more, though it really isn't.

My favorite portrait focal length was 200 mm, for a couple of reasons.
1. It kept me well outside a customers 'personal space'.
2. I loved that there was less in the background because it was magnified, and the way out of focus background elements were magnified and made to seem more blurred.

To keep subject scale the same you have the camera further from the subject with an 85 mm than with a 50 mm.
With the increase in point of focus distance the DoF remains almost exactly the same for each lens aperture used.

You can check that using an online DoF calculator.
A 50 mm lens at f/1.8 on a crop sensor DSLR and a PoF distance of 10 feet gives a total DoF of 0.81 feet.
An 85 mm lens at f/1.8 on a crop sensor DSLR and a PoF distance of 17 feet, for the same subject scale in the image frame, also gives a total DoF of 0.81 feet.

 

[soufiej]

If I have a prime lens with 50mm focal length and 1.8 max aperture, and a lens with 85mm focal length and 1.8 max aperture, what differences are there besides the zoom provided by the differing focal lengths? I know that 85mm, 1.8f is preferred for portraits for some reason. Is there some other special effect one lens might have that the other does not?

One way to learn about a lens is to read about a lens. While reading, it is important, IMO, to differentiate between objective and subjective values. Objective values are those things someone can measure and place a quantitative value on. A lens with image stabilization is different than a lens without image stabilization. That is an objective value. Obviously, IS is not required for a lens to be measurably good and its presence can, in some instances, actually be a hindrance to the lens being good for actual use. Widest aperture, fastest shutter speed, least amount of chromatic abberation, etc are all objective values which can be tested.

These objective measurements inform you as to how the lens performed in a "lab test situation". Lab tests exist under strictly controlled conditions, nothing varies from one to the next and, supposedly, the tests are designed to give each lens the best advantages possible when being tested. Lighting conditions, distances and so forth are all measured and maintained. the lens is mounted on a "superior" camera and then the camera has been mounted on a tripod and secured in order to ensure the lens does not move - or moves as minimally as possible - during shutter release.

Unfortunately, the vast majority of real world photography does not exist in a lab. Conditions of use are not strictly maintained. It can be said in those cases, that the lab test is not really very important to the actual use. Therefore, many real world users consider objective measurements to be only a small aspect of how a lens performs. Though it must be said, how a lens performs for one photographer may not be how that same lens performs for another. The many variables which go into an interchangeable lens system make highly controlled lab tests a bit of a joke in some cases. We each have our own priorities regarding subjective values and those are what will guide us in actual real world use. A 400 mm zoom would be rather out of place for a portrait photographer so the quality of the 400 mm lens isn't even a factor in the portrait photographer's considerations. If you are not photographing vast landscapes, do you truly need that new 11 mm lens? Probably not, so it's value to you as a photographer is greatly diminished.

Beyond that though, when you read someone on, say, a forum saying how wonderful their new lens is, keep in mind one fact. Everyone loves what they just purchased, until they don't. People prefer to find the good in something before they realize every thing they buy has trade offs and what they just purchased is no exception. Eventually, many of those in love with their new acquisition will discover the trade offs and possibly no longer care quite so much for a product. Some sooner than others; there's a great deal of brand loyalty involved and many buyers like to think something is "just as good" if it is less expensive. So applying subjective values can be dangerous if you only take people at their word today. That said, ...

Subjectively (in other words, from the viewpoint of any one individual), how the lens feels and its mechanical operation are of prime importance. What good does it do for a lens to be extremely "fast" if there are significant distortions present when using the advantage of light gathering? If a lens is "sharp" as measured in a lab where time is not of any great consequence, what does that matter to the user in the field who must make on the fly adjustments virtually in an instant?

IMO, the difference between the objective view and the subjective view is a bit like the difference between pasta which has been cooked according to package directions (the objective veiwpoint) and pasta which has been cooked al dente and with just the proper amount of sea salt added to the water (the subjective viewpoint). Try to find an Italian restaurant that doesn't over cook their pasta. People are rather dumb and they prefer (objectively) overcooked pasta so that's what they get, even at the high end joints. IMO the two are as different as being cooked and being satisfying. There is no objective measurement for satisfaction (the subjective viewpoint).

Products can be flawed in some manner yet be so significantly more satisfying than another that the flaws can be overlooked for the satisfaction derived in the use. One product can be objectively "accurate" yet be extremely boring in use. Another may measure as less accurate but create something which has beauty and makes the user more interested in the results. Therefore, my advice when reading lab tests would be, take them with a great many grains of salt and don't overcook them; Lenses hub Digital Photography Review

However, since there are many objective measurements which go into making a good lens on paper, reading how lenses should operate is where you begin learning about what makes one lens different and possibly "better" than another.

 

[astroNikon]

I know that 85mm, 1.8f is preferred for portraits for some reason. Is there some other special effect one lens might have that the other does not?

It's more related to when you increase the focal length at some point the image gets less distorted.
There's plenty of pictures on the internet of this issue such as here ==> Which Lenses Are Best For Wedding Photography and Why Focal Length Matters Syracuse Wedding Photographer John Carnessali

 

[Derrel]

Focal length changes how large an image is created. With a subject at say, 10 feet, with the 50mm lens the subject will be of one size; using the 85mm lens, the same subject will be rendered larger from that 10 foot shooting distance. Secondly, the longer the focal length, the narrower the picture angle is. Also, the longer the lens is, the more the BACKGROUND items are literally "magnified" in size.

A wide-angle lens appears to make the background appear farther away, and literally wider, than a normal lens would make things appear. A telephoto lens makes the background larger, and narrower, and appears closer, than a normal lens would.

A normal focal length lens is approximately as long as the diagonal measurement of the sensor or film being used, and a normal focal length lens creates pictures where the background appears neither unnaturally far away, nor unnaturally close.

Extreme wide-angle lenses, and extreme or super-telephoto lenses both create very astonishing renderings of real-world scenes, with very distorted spatial and size relationships. Lenses like 8mm and 10mm fisheyes and 14mm ultra-wide (rectilinear) lenses and 400,500,and 600mm lenses are interesting speciality tools.

 

[petrochemist]

The 50mm will give close to the viewpoint your eyes give, whilst the longer focal length of the 85mm encourages you to move further back, giving lessened perspective.The more distant perspective is generally much more flattering.
I suspect the classic 85mm as the optimum portrait focal length comes down to the distance we usually interact with others at, which may be why our minds ind this degree of perspective comfortable. Although people do sometimes talk close enough to only see the faces of each other most of the time a little more personal space is preferred, so they'll be that little bit further back than the 50mm position. If doing full length portraits the 50 works just fine.