I've a Pentax K-30 with the following lens:

  • Pentax-M 50mm 1.7 manual focus

  • Tamron 17-50mm 2.8 autofocus

  • Pentax 55-300mm 4-5.6 autofocus

Considering that :

  1. Pentax 50mm has the nicest bokeh and sharpness I've ever see but, manual focusing, is prone to error
  2. and 55-300 has the 'ideal' focal length for shooting portrait (for ideal i mean 70-90mm), autofocus, but it hasn't good bokeh

Which should I use?

  • how well lit is your subject? do you have a reason to need a zoom lens?
    – Skaperen
    Aug 24 '15 at 9:05
  • The reason is i've read in a lot of book / photography site, that for portrait a good focal length is 70-105mm and a lot of photographer use a zoom lens (Kelby for instance, or here photo.stackexchange.com/questions/15732/…)
    – stighy
    Aug 24 '15 at 9:09
  • Take the lens that give you the "best" image (yes, relatively to you) ?
    – Olivier
    Aug 24 '15 at 9:58
  • But because i'm a beginner, maybe i think 50mm give me the best image, but a 70-100 mm focal length or (someone says) 200mm give better image...
    – stighy
    Aug 24 '15 at 10:16
  • 3
    Note that it appears to be a crop frame sensor in this camera according to DP Review (link), so any lens focal length would be x1.5, so the 50mm lens (and the top end of your Tamron autofocus lens) would actually be equivalent to 75mm, within your ideal portrait focal length Aug 24 '15 at 11:56

Use 50mm if you want to separate the person from the bacground, you can always crop the image to simulate the 70mm (at 8Mpix) or 90mm (at 5Mpix). To include enviroment use 17-50/2.8 at smaller focus distances. And take telezoom for larger distances.

You may found usefull to experiment with http://camerasim.com/apps/camera-simulator/


If you are going to make mostly portraits, a 50mm prime would fit you much better. Not only gives you a lower aperture, but the image quality is better with primes.

Don't worry about the "ideal" 70-90mm range, 50mm is a classic focal length for portraits.


It also depends on how much background do you want to keep.

With the Pentax prime or Tamron zoom lens, you'll get a better background separation, in addition to having more backdrop behind your subject. This will arguably be nicer as the bokeh effect will be more apparent (more concentric circles, soft focus etc.).

On the other hand, if you want to get closer to your subjects without being too literally in their face, this is where the longer focal length will come in handy. In addition, any prevalent pincushioning lens distortion tend to be more flattering too, as it makes subjects just that little bit slimmer...


I think that taking a head and shoulders picture with a 50mm lens means that you have to get too close for your subject's comfort and in addition risk some distortion.

I've always used 90mm prime lenses with as fast an apeture as my bank balance will permit. As usual with photography, it's a trade off - it's a great lens but I have to move around a good deal to recompose the shots. Obviously, with a zoom I wouldn't have to do this but really fast zooms aren't cheap.

The bottom line is that you should be fine with your zoom shooting at f5.6.

  • 1
    Are you hanging that 90mm lens on a FF or APS-C body? The OP is using an APS-C body, which means the 50mm lens gives the FoV of a 75mm lens on a FF.
    – Michael C
    Aug 24 '15 at 20:58

(I am teasing you:) You seem to be seeking a hard rule that you can apply without thinking, but there are no hard rules, certainly no One rule, and thinking is always helpful. :) It always depends, on the situation, and on what result you want.

70-90 mm is "ideal", if assuming a cropped APS sensor, and assuming a normal subject distance of at least 6 to 8 feet (for proper portrait perspective), and it is not a group shot. But even better, the 70-300 will also do say, 150 mm or 200mm, which from say around 15 or 18 feet (2x focal is 2x distance) is still ideal, still with proper perspective.

And (speaking about outdoors) if focused at this 15 feet, it will blur the dickens out of any background that is reasonably distant. Plus, as important, it sees a much more narrow view of the background, so that moving slightly can easily choose a better one.

IMO, beginners go for the f/1.8 lens, because they are inexpensive, and they heard they should, (do you really like f/1.8 portraits?), and because they normally stand too close for perspective anyway.

IMO, pros probably more often seek the longer lenses from the longer distances, which are not f/1.8 (another plus), and they do the job better, good perspective, and better background choice.

  • The 'pro portrait lens' may not be f/1.8. Instead, its f/2.0. For example, the Nikon f/2.0 105mm DC and the Nikon f/2.0 135mm DC or in the Canon world, the 135mm f/2.0L . There's also the 85mm f/1.8 and 85mm f/1.4 for people who like even faster lenses in the classic portrait range.
    – user13451
    Aug 25 '15 at 21:50

Pentax 50mm has the nicest bokeh and sharpness I've ever see...

I think you've answered your question. Stick with image quality over anything else. You can always make up for other deficiencies with skill and technique. Shoot for best image quality.

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