Blackmagic Forum

Hey,

Yesterday I started a product shoot with Sigma 18-35 and Sigma 50-100mm. I shot them both at the same aperture (f8) with the same amount of light but the 18-35 was darker ( I shot at 35mm and 100mm).

It's a moody, low key light set up so the difference in that context was quite extreme.

I assumed that the two lenses would match perfectly which was why I got them.

Forgive me if this is an obvious question but is it normal for a longer focal length produce a brighter image and is there a rule of how much to compensate? Due to it being a product shoot I need the images to match exactly straight out of the camera.

I've typically done run and gun documentary work outside using prime vintage lenses so this isn't something that's become apparent to me until now. At first I thought maybe there was something wrong with the 18-35! Any help/thoughts much appreciated.

Did you use the still or the cine versions of those lenses?

If the images are different the stop isn’t the same. The question is why is the lens or camera reporting the information differently? Are both lenses on the Ver 2.03 firmware?

“ ….match exactly straight out of the camera.” Given this expectation “quite extreme” could be understood as a quarter stop or less. You’ll get better feedback with pics and data.

Good Luck

Howard Roll wrote:If the images are different the stop isn’t the same. The question is why is the lens or camera reporting the information differently? Are both lenses on the Ver 2.03 firmware?

“ ….match exactly straight out of the camera.” Given this expectation “quite extreme” could be understood as a quarter stop or less. You’ll get better feedback with pics and data.

Good Luck

Hey Howard, see that attached image taken just now - deliberately overexposed to demonstrate the point. The one on the left is the 50-100mm, the one on the right is the 18-35mm.

I have no idea about the firmware, I've only ever used vintage lenses where this wasn't a factor. Just googled it and looks like I need to get a USB dock to check?

Yeh, more than a stop. So the 50-100mm is consistently brighter? What stop was this? Do you have what would appear to be the full range of iris control on both lenses? I'd consider this difference to be larger than what would be expected from trusting manufacturer reported f numbers or the variance from differing lens control protocols. A few decimals either way sure, not this.

Long story short, something is wrong, there's no rule or principle that will compensate for this kind of shift.

At minimum I'd shoot a test at f1.8 and f16 and compare the 4 images to isolate the problem lens. If the image is not clipped or crushed there will be better data for analysis, when the picture is clipped as above it makes critical comparison challenging.

Good Luck

Howard Roll wrote:Yeh, more than a stop. So the 50-100mm is consistently brighter? What stop was this? Do you have what would appear to be the full range of iris control on both lenses? I'd consider this difference to be larger than what would be expected from trusting manufacturer reported f numbers or the variance from differing lens control protocols. A few decimals either way sure, not this.

Long story short, something is wrong, there's no rule or principle that will compensate for this kind of shift.

At minimum I'd shoot a test at f1.8 and f16 and compare the 4 images to isolate the problem lens. If the image is not clipped or crushed there will be better data for analysis, when the picture is clipped as above it makes critical comparison challenging.

Good Luck

That was at f1.8.

The aperture is changing on both lenses as I change it on screen, but the f1.8 just doesn't seem bright enough on the 18-35mm for how that fstop should look.

I've attached images at a few different f stops and iso's for reference. The difference is most noticeable at f1.8 and least noticeable (though still there) at f16.

The 18-35mm doesn't seem to be keeping up. Even at F8 it's a couple stops behind. You're probably running into an issue similar to what these folks were experiencing. It may be time to roll the firmware on your cam.

viewtopic.php?f=2&t=179297&p=1002546#p1002546

Good Luck

Howard Roll wrote:The 18-35mm doesn't seem to be keeping up. Even at F8 it's a couple stops behind. You're probably running into an issue similar to what these folks were experiencing. It may be time to roll the firmware on your cam.

viewtopic.php?f=2&t=179297&p=1002546#p1002546

Good Luck

Hmm strange, I'm on the latest update of Blackmagic whereas those in that thread had their issues solved by updating. Perhaps it's a lens firmware update as you mentioned, I'll have to buy a docking station and check

kyanmagic wrote:

Howard Roll wrote:The 18-35mm doesn't seem to be keeping up. Even at F8 it's a couple stops behind. You're probably running into an issue similar to what these folks were experiencing. It may be time to roll the firmware on your cam.

viewtopic.php?f=2&t=179297&p=1002546#p1002546

Good Luck

Hmm strange, I'm on the latest update of Blackmagic whereas those in that thread had their issues solved by updating. Perhaps it's a lens firmware update as you mentioned, I'll have to buy a docking station and check

Look for a second hand one, you might use it one time for each lens. If you were in Spain I’d have lent you mine. I bought it second hand and used it only one time for my sigma 12-24.

kyanmagic wrote:Hey,

Yesterday I started a product shoot with Sigma 18-35 and Sigma 50-100mm. I shot them both at the same aperture (f8) with the same amount of light but the 18-35 was darker ( I shot at 35mm and 100mm).

It's a moody, low key light set up so the difference in that context was quite extreme.

I assumed that the two lenses would match perfectly which was why I got them.

Forgive me if this is an obvious question but is it normal for a longer focal length produce a brighter image and is there a rule of how much to compensate? Due to it being a product shoot I need the images to match exactly straight out of the camera.

I've typically done run and gun documentary work outside using prime vintage lenses so this isn't something that's become apparent to me until now. At first I thought maybe there was something wrong with the 18-35! Any help/thoughts much appreciated.

SOLVED - It was a firmware issue. Updating the firmware with the USB docking station solved the problem

Yesterday I made some tests shots with three lenses (don't have the 50-100 still lens):

Sigma 18-35mm f1.8 DC
Sigma 18-35mm T2.0 Cine
Sigma 50-100mm T2.0 Cine

Set the aperture on the Cine lenses to T8.0 which equals f/7.3 and made a second shot with stops closed until camera showed f/8.0.

Highres Tiffs can be downloaded here:
https://we.tl/t-zWFbtJ0GXI