Actual F-Stop for Rokinon 85mm/1.4 ???

My recently purchased Rokinon 85mm f/1.4 lens appears to deliver only about f/1.8

I determined this by comparing histograms for manual exposures at f/1.4, f/2, and f/5.6. The histogram for 1/320s @f/1.4 showed underexposure when compared to 1/160s @f/2 or 1/20s @f/5.6., both of which were the same. To get the same histogram at f/1.4, I needed to drop down to 1/200s, a loss of 2/3 f-stops. I also tried two other canon lenses at 1/20s, f/5.6, and their histograms matched the Rokinon at the same f-stop.

I found this review at http://www.bobatkins.c​om …/pro-optic_85mm_f1-4.html. It suggests that the maximum aperture of the Bower (same lens) was about f/1.6.

Although I like the lens, I find it hard to focus manually, except in live view. Its image quality is very good, provided I get the focus right. I am, therefore, thinking about sending it back, to be replaced by either the Canon 85mm/1.8 or 50mm/1.4. Although either is more expensive than the Rokinon, they would give me autofocus and, hopefully, deliver the advertised aperture.

has you tried a EF-S focus screen? good manually focusing at f1.8 without it...

well the F is 1.4, what you're measuring is T stops. Sensors cant absorb all of that light transmission:
http://www.dxomark.com​/Reviews/F-stop-blues
canon secretly boosts iso for faster lenses like F1.2.

you can see here the actual Tstop values of similar lenses: http://www.dxomark.com …8-USM___363_0_270_0_241_​0

Rodney - how far away was the subject when doing the test? If you were working at a close distance, the focal length may not have been as advertised. Most lens' focal lengths are stated only for "infinite" focus. If the focal length for your tests was something other than advertised, maybe the f-stop calculation changed as well.

Charlie wrote in post #16997803
well the F is 1.4, what you're measuring is T stops. Sensors cant absorb all of that light transmission:
http://www.dxomark.com​/Reviews/F-stop-blues
canon secretly boosts iso for faster lenses like F1.2.

you can see here the actual Tstop values of similar lenses: http://www.dxomark.com …8-USM___363_0_270_0_241_​0

This.

Also, there's way more value to the Rokinon than just being an f1.4 lens. I'd have kept mine over the Canon 1.8 (I had bought both and returned the Canon) even if it's max aperture was only f2.

I thank the respondents for some good ideas. I checked the DxOMark link suggested by EverydayGetaway which showed actual T-stop for verious lenses. My Rokinon came out at f/1.7, the Canon 85/1.8 at f/2, and the Canon 50/1.4 at 1.6.

I also visited another DxOMark link by Charlie. It showd that digital sensors aren't very good with fast lenses. And sensors with smaller pixels, i.e. crop sensors like my 70D, are more adversely effected than full frame sensors with the same number of (larger) pixels. It may be that lenses faster than about f/1.8 are effective only on full frame.

SkipD suggested that focusing distance might be a factor. I don't think so. My tests were done with a mostly blank wall at about 15 feet. But I may repeat the tests at a greater distance anyway.

Finally, venom3300 suggested an EFS focusing screen. But Canon doesn't make one, although I could try third-party solutions.

I do want to keep the lens. It is relatively light, inexpensive, and sharp. Samples on digital-picture.com suggest that it sharper, wide open, than just about any comparable 50mm or 85mm lens for Canon except the Sigma 50/1.4ART. After reading the responses to my post, I think I will keep the lens.

Heya,

I have the Samyang 85mm F1.4 and the Canon EF 85mm F1.8.

Samyang F1.4 at F1.4:

IMAGE LINK: https://flic.kr/p/nXMk​7w

IMAGE LINK: https://flic.kr/p/nkWz​dB

venom3300 wrote in post #16997677
has you tried a EF-S focus screen? good manually focusing at f1.8 without it...

Thanks for the suggestion, I tried it works!

But first I tried live view with 10X zoom on my 70D. This increased my hit rate considerably. But it worked only for stationary subjects, and only if I was careful not to lean forward or backward after achieving focus and before taking the picture.

Next I tried Magic Lantern on my 60D, using "focus peaking". This worked better, provided I achieved near-focus before activating focus peaking. And, with my non-chipped lens, I still had to use live view.

Finally I ordered the EF-S focusing screen from Amazon.com. (The Canon website showed it "out of stock".) After installing it in my 60D, I got a higher hit rate, provided I had something in my composition with sharp detail. And I didn't have to use live view.

Note - Magic Lantern does not support my 70D. And, although the Canon website lists the focusing screen as 40D only, it apparently also fits the 50D and 60D, but not my 70D. So I will dedicate my 60D to the Rokinon 85.

sorry for hijacking this thread,i have a 60d with a rokinon 1.4 that i just got,i too struggle with focus wit this combo,what is a focus screen and how does it help with focus,thanx in advance

The older manual focus cameras had a split image rangefinder and micro-prism collar which helped with focus and most modern AF cameras lack. The electronic viewfinder models (and some monitors) often have false color focus assist.

I have the same lens in the cine version which is demarcated in T-stops and the widest opening is 1.8.

genesimmons wrote in post #17058040
sorry for hijacking this thread,i have a 60d with a rokinon 1.4 that i just got,i too struggle with focus wit this combo,what is a focus screen and how does it help with focus,thanx in advance

In an SLR (or DSLR), when the mirror is down, the image is reflected up to a flat surface called a focusing screen. In the days before autofocus SLR's, these screens were designed to make focusing easier. Usually they had a "split image" focusing aid. These would split the image. The two halves of the image would line up only when the image was in focus. The older screens would also have an area that was roughened a bit to make it more obvious that the image was in or out of focus.

Unfortunately, modern autofocus SLR's assume that their users would never need to focus manually, and have focusing screens with a relatively smooth surface that makes the image a bit brighter but does nothing for manual focusing.

The Canon EF-S screen does not have the split image aid, but is slightly roughened to make focusing easier. With it, the image pops in and out of focus more decisively than with the standard focusing screen.

The last time I checked, the screen was "out of stock" on the Canon website, but Amazon.COM has a few left at ..

http://www.amazon.com …age_o01_s00?ie=​UTF8&psc=1