rcirr Loc: Gilbert, Arizona

Just received a Rokinon 14 mm, F2,8 AF lens for Christmas. The lens was bought off my wish list. I selected the lens strictly by specs. I put the lens onto my Nikon D750. Shot a few manual, a few auto, a few "S" mode, and a few "A" mode. So far it worked as expected. I attached a flash to my camera and tried a few shots (manual mode, Auto mode, "A" mode and "S" mode. In every non manual mode (any time the camera controlled the flash) the results were extremely over exposed ( 4.2 F Stops). This happened with both my Nikon speedlight(SB-600)as well as my Yongnuo speedlight (YN568EX). If I set the flash to compensate 4.2 F stops, the pictures came out fine. If not, the picture was basically a white screen. I can't find any explanation. Any one have a clut?

Orphoto Loc: Oregon

Take a quick look at the flash units to make sure they dont have any exposure compensation dialed in.

Bridges Loc: Memphis, Charleston SC, now Nazareth PA

I've experienced that with some lenses also. The lens may not have the right electrical tabs (that communicate with the camera) to tell the flash what the lens's focal length is, therefore confusing the flash as to how much output it needs. If you notice, often a flash will display the lens' focal length on the info screen on the back of the flash. Also with such a wide-angle lens, you may be so close to the subject that the flash even at a low setting is too much.

sippyjug104 Loc: Missouri

I agree with Bridges and I believe that he is on point for the intensity of light varies to the square of the distance from its source.

User ID

4.2 stops ? You know the degree of over expisure to within a tenth of a stop so you must have the exif. How does the aperture in use relate to the resultant image as you change from auto to manual ?

Shutter speed would not matter unless somehow the shutter is delivering about 1/4sec when ambient exposure should be about 1/60 ... and that might hint that this lens somehow pollutes your cameras data stream.

The lens firmware is possibly reverse engineered rather than licensed which does lead to mysterious behaviors. I had some wonderful Sigma lenses on my EOS film cameras but the same lenses will shut down any digital EOS that they get mounted to. Acoarst Canon EOS lenses from that era cross over to digital with zero hiccups.

larryepage Loc: North Texas area

While I am not familiar with the specific Yongnuo flash you are using, I know of very few flashes that will properly cover the field of view of a 14mm wide angle lens.. i know that my SB-800 flash, which has a variable reflector system, will only cover the widest lenses if the accessory white diffusion dome is installed. Installing this dome also activates a switch which makes adjustments to the automatic exposure system.

Have you verified that your flashes are capable of covering such a wide field? (I doubt that the SB-600 is.)
Have you verified that you have made any necessary adjustments to allow it to do so?

Even if the speedlight is covering the desired area, it is possible that the auto exposure algorithms fail at the short focal lengths. And just as a quick check...when you do an image review, is your lens being properly reported as 14mm?

MrPhotog

Only time I’ve had a similar problem was when my ISO was set high (3200) for available light shots, and it was more than the flash could handle. Lowering the ISO to 200 put my camera in the range where it could control the flash properly.

What ISO are you using?

User ID

Why would FL matter ? FL just crops the image, nothing to do with intensity or distance from source.

larryepage Loc: North Texas area

Because a number of flashes have the capability to reconfigure the reflector in the flash head to match the focal length of the lens. Doing so changes the intensity of the light. If it changes to an unmatched FL, bad things can happen. In this case, the OP is using a third party lens. We need to know that it is reporting its focal length correctly. Exposure at 14mm would be a little less than 4 stops different from what it would would be if the same light only had to cover a 50mm fiels of view. Of course, we would be expecting underexposure, not the overexposurr reported by the OP. Bottom line...something's not working right and we are trying to help diagnose it.

User ID

That doesnt address the question. Dearth of coverage in the corners will not cause anything like "4.2 stops" of overexposure and in most systems no effect at all. As you clearly advised, any effect would be underexposure, not the opposite blown out overexposure.

You are ignoring that it all works well in manual exposure. Zoom flash heads dont stop zooming just cuz the user is setting the exposure. You hafta go into the menu and cancel the auto zoom.

Theres nothin unusual here. The obvious problem is lens FW and short of some update the obvious solution is user control of exposure.

larryepage Loc: North Texas area

Or just that the Rokinon lens is not communicating completely with the camera...

User ID

Yes. Not completely or not correctly. Thaz just the two faces of the same dime.

Theres a pretty good chance I have some mismatched gear that could cause weird results for some users. I may never know about it cuz I set things manually ... not so I can brag about it but just cuz I hate unfortunate surprises (even all-OEM isnt perfect, just safer).

ArcticTrails Loc: Valdez Alaska

I don’t think the focal length of the lens is as important to exposure as the distance the lens if focused at. Remember the old Nikon D lenses they were the first t report the focus information back to the camera for this very purpose. Take a look at your EXIF data and see if the focus distance is reporting accurately.

Cheers

Grahame Loc: Fiji

I have an old Samyang/Rokinon 14mm f/2.8 non AF with Aperture ring and along with a D800 undertook some testing to see what happened with my SB-600. This lens gives no Aperture or distance feedback to the camera. Whilst not exactly the same setup it may give some clues.

The SB-600 was set to TTL with 0FC and it's minimum zoom of 24mm. The camera was set to TTL, ISO400

In Manual mode with Speed set manually to 1/60s and with the lens at both f/2.8 and then f/22 exposure (brightness of the subject) was the same, and was good. The subject was approx 1m away, there was light fall off to the sides due to the 14mm and this can be slightly improved by using the pop out wide screen panel.

In Aperture Priority, Shutter Priority and Program mode the camera speed was set to 1/60s when the flash was turned on. The same tests at f/2.8 and f/22 were undertaken in each of the three modes with near as identical results as when in manual.

The above demonstrated that with this lens that does not communicate aperture or distance the TTL controlled the exposure to within say 1/3 stop of ideal. In addition, in each of the tests I also took a shot at +2FC and got exactly what I would expect.

BebuLamar

I wonder what kind of aperture the OP used? I find it's hard to overexpose my flash shots and I used SB-800 and Metz 60CT4. I always feel I need more flash power not less.