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12-02-2013, 04:25 PM   #1
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Flash Photography with K-50 & SMC-M 50mm Lens

I've attached a Pentax 50mm f/2.0 Manual lens to my DSLR. Everything seems to work OK except when I use the built-in flash: the camera doesn't seem to be compensating this makes sense because pressing the green button starts the metering but the flash doesn't fire at that time and the photos are entirely washed-out. Since the camera automatically chooses the shutter speed (I have it set to 'M' mode as per instructions) I haven't a clue as to how to do flash photography with this lens. Help is greatly appreciated.

12-02-2013, 04:55 PM - 2 Likes   #2
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That is normal, the P-TTL flash system won't work without a lens that has A contacts for exposure. It will just fire full power every time. You can get around this by stopping down, the GN of the onboard flash is 12 meters at 100 ISO. The formula to calculate the aperture you need is f = GN/distance. So at 1.5m away you'd want the aperture to be f8 (12/1.5), with the subject 3m away you'd be at f4 (12/3) and so on. If you're using 200 ISO just double the GN, ISO 400 double it again and so on. If you are more comfortable with feet instead just use 40 for the GN and estimate distance in feet.

The K-3 introduced manual control for the onboard flash to help with this. That way you can turn down the flash power for closer subject you might want to shoot at a wider aperture.

Your best option is to get a hotshoe flash, especially one with the auto mode that has its own light sensor.
12-02-2013, 05:55 PM   #3
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Thank you for the excellent explaination, Elliott. I have a feeling I might be looking at SMC-A primes sometime in the future, but for now I'll be using the 18—55mm for fast action shots. Doing a calculation followed by an adjustment isn't going to cut it when trying to capture cats at play!
12-02-2013, 06:10 PM   #4
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There are ways to do it.

Pre-focus the lens at a reasonable distance, I think 3m would get you a decent framing on something the size of a cat at 50mm. Then setup your exposure to match this distance, you'll want a small aperture so go for 400 ISO and f8 if you are focused at 3m. When they are in focus (or close enough, f8 gives you plenty of DOF) just hit the shutter.


Hotshoe flashes with auto mode simplify this a lot. Just match the aperture and ISO on the camera to the aperture and ISO the flash is expecting and you don't have to worry about distance at all, just shoot.

12-02-2013, 09:33 PM   #5
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Great suggestions. I knew about setting the focus, but now I know which aperture settings to use. But before I read your reply I 'gave in' and attached my DA 18–55mm f3.5–5.6 to the K-50, clipped on a Gary Fong Puffer (an inexpensive light diffuser), popped my flash, and came up with this taken in very low light:
Untitled | Flickr - Photo Sharing!
The only changes to the original DNG raw file were cropping and red-eye reduction done in Lightroom. I admit that being able to use the flash this easily was a pure joy.
12-03-2013, 09:36 PM   #6
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I'm trying to figure this manual lens and flash thing out as well.
I'm enjoying my 50mm 1.7 manual with the green button for metering and auto shutter. But, using the on board flash is resulting in extreme over exposure.

I understand the aperture selection using f = GN/distance. Do I just skip the green button metering and set a shutter speed inverse to my ISO?
12-03-2013, 09:57 PM   #7
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Your shutter speed depends on what you want to do with the ambient light. Slower shutter speeds will give you more ambient light in your photos, but will not affect the flash exposure at all. The fastest you can have is 1/180s.

Sticking to the numbers earlier, if your subject is at 3m and your ISO is 400, then your aperture will be f8. Now if there is a large room behind your subject that is lit at about 10EV and will not be lit by the flash, then you could set a shutter speed of 1/60s and the room would appear to be normally lit and your subject will be properly lit by the flash. Faster shutter speed would make that room appear to be darker. So you can just green button meter to get an appropriate ambient exposure or underexpose the ambient if you want to.

Using the pop up flash without P-TTL is no different from using a manual hotshoe flash, the concepts are all the same, you just can't change the output of the pop up flash unless you have a K-3.

Last edited by elliott; 12-03-2013 at 10:58 PM.
12-03-2013, 10:54 PM   #8
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QuoteOriginally posted by elliott Quote
That is normal, the P-TTL flash system won't work without a lens that has A contacts for exposure. It will just fire full power every time. You can get around this by stopping down, the GN of the onboard flash is 12 meters at 100 ISO. The formula to calculate the aperture you need is f = GN/distance. So at 1.5m away you'd want the aperture to be f8 (12/1.5), with the subject 3m away you'd be at f4 (12/3) and so on. If you're using 200 ISO just double the GN, ISO 400 double it again and so on. If you are more comfortable with feet instead just use 40 for the GN and estimate distance in feet.

The K-3 introduced manual control for the onboard flash to help with this. That way you can turn down the flash power for closer subject you might want to shoot at a wider aperture.

Your best option is to get a hotshoe flash, especially one with the auto mode that has its own light sensor.
Thanks Elliott, thats some handy beta. My K-50 shipped today.

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