Which Prime for a Full Frame camera

[Beggarall]

I am looking for some help and guidance. I have a Sony A7 camera with a 24-70 F4 lens but am struggling to get auto-focus in dark/dim light. It seems that the best way to go is to buy a prime lens which will allow wider aperture but these are quite expensive!! I wondered which would be my best choice - 35, 50 or 85mm (or something else). I find the 24-70 covers most of what I like to do so slightly discount the 85mm lens. Anybody any experience with Sony lenses? Thanks in advance.

[WWA]

I have the 24-70 and the 35mm and 55mm. The Zeiss 55mm is a lovely lens with beautiful results, the 35mm is very close to being as good. Work out which focal length you use most when struggling in low light and go for the nearest equivalent prime. The 55 is 1.8 and the 35 is 2.8 so this could matter to you depending on what you shoot. Hope that helps.

[ukaskew]

The 55mm 1.8 is ridiculously good and will be a bit of a low light monster compared to the f4 zoom. The 35mm 2.8 is a good lens, but it's only one stop faster than your zoom, so you won't see a huge benefit.

I loved mine, very compact and fantastic image quality https://www.flickr.com/search/?user_id=43894860%40...

The new Sony 50mm 1.8 FE would also worth be considering as it's less than half the price of a 55mm 1.8 (it's about £240).

Edited by ukaskew on Thursday 8th September 08:57

[andy-xr]

Phase detect focus or contrast based? You might be better buying a torch tbh

[kman]

autofocus in low light isn't really helped that much by the lens max aperture - in most cases superfast primes are typically slower to focus than zooms and the dof is much shallower too resulting in missed focus more often. Might be an idea to buy a flashgun, and use IR focus assist from the flashgun in extreme lowlight conditions.

Edited by kman on Saturday 10th September 01:35

[kman]

explained here (for nikon speedlights not sure how sony ones work):

https://photographylife.com/how-to-make-autofocus-...

[Simpo Two]

kman said:

explained here (for nikon speedlights not sure how sony ones work):

https://photographylife.com/how-to-make-autofocus-...

Interesting idea. I used SB800s and SC-29 synch cord, both of which have IR assist. With these I could have taken well-exposed, sharp photos in total darkness (had I wished to!). That was using flash of course; if you only want the focus assist but not the flash, you might also be able to achieve it by setting one of the function buttons to 'disable flash'. I used this often with my D200, but not to disable the flash after it had acquired focus. Worth trying though.

Going back to the plot, a faster aperture will halp AF systems work in low light BUT they still need light and contrast to work.

[kman]

Simpo Two said:

Interesting idea. I used SB800s and SC-29 synch cord, both of which have IR assist. With these I could have taken well-exposed, sharp photos in total darkness (had I wished to!). That was using flash of course; if you only want the focus assist but not the flash, you might also be able to achieve it by setting one of the function buttons to 'disable flash'. I used this often with my D200, but not to disable the flash after it had acquired focus. Worth trying though.

Going back to the plot, a faster aperture will halp AF systems work in low light BUT they still need light and contrast to work.

If shooting still life I agree a faster aperture will help. Shooting moving people in the dark - in practical terms its more difficult - most primes can't shift the glass elements around as quick as the motor can on a zoom but it depends on the lens - my 85mm is slow, my 24mm is pretty close in speed to my 24-70 and my 35mm again quite slow and prone to hunting. Its not just the amount of elements in the lens that counts but also the focus throw (from close-up to infinity). The zooms again have hardly any travel to do. Saying that my fastest lens (in AF terms) was my old 200mm f/2 nikon which was ultra-quick!

Yes whilst my bodies are quite impressive in locking focus in low light (they supposedly work up to -4ev), there's a point beyond which they really start to struggle, regardless of what lens is on the camera. Thats where IR assist really comes in - it can lock focus on faces and get sharp images where I can't even see the face through the viewfinder - im literally guessing where the subject eyes are and putting my focus point over it and it works. In that scenario, I need the flash firing anyway. The only drawback is its single shot AF only, and can only use the center focus points (as the IR beam goes straight out).

[Simpo Two]

kman said:

most primes can't shift the glass elements around as quick as the motor can on a zoom

Ar you sure? A zoom has more glass/elements to move than a prime. My 50mm f1.4 is AF-D so not fast to focus but I'm sure an AF-S version would be as quick, if not quicker, than a zoom.

[kman]

Simpo Two said:

Ar you sure? A zoom has more glass/elements to move than a prime. My 50mm f1.4 is AF-D so not fast to focus but I'm sure an AF-S version would be as quick, if not quicker, than a zoom.

yep done numerous real world tests and I find the fastest lens ive used in terms of AF speed would be as below (fastest to slowest):

Nikon 200/2G
Nikon 24-70G
Nikon 70-200G
Nikon 24/1.4

^ all pretty fast to focus

Slow:

Nikon 35/1.4, 85/1.4 and 50/1.4G are slow.

I'm told the Sigma ART series equivalent primes have quicker AF but not tried them myself.

I still use my 35 and 85 for documentary shooting where i can as they are my favourite lens but if im shooting on a dance floor up close they have no chance so out comes the 24-70 or 24mm.