Depth of Field/Aperture/Flash

[kirbym2]

Having a terrible time wrapping my head around this concept. Most lenses seem to be their sharpest when stopped down a few from their widest aperture (I hope that's not a misnomer too). If I'm wanting tack sharp portraits isolated from the background, is there a resource where I can calculate distance from camera to subject, and subject to background?

I think I've got the concept that the further a subject from the background, the better time I will have isolating it (the subject).

Any portrait photographers or photographers in general have a recommending aperture setting when shooting couples, families, etc?

Another curveball seems to have been thrown my way when it comes to using flash, and aperture to control flash exposure. Should I be choosing the aperture I want to be working with first, and then playing with the distance between the subject and flash? I understand there can be quite a variance in distance - should I be aiming for the 1/1 power output range on the flash, or reduce the power (i.e. 1/4, or 1/8).

Loaded topic... any help would be much appreciated!

 

[clanthar]

Online Depth of Field Calculator

 

[Big Mike]

You will usually want enough DOF to encompass your subject, especially when shooting a group. Although, if you're shooting a single person, it may be enough to have just their eyes in focus (if that's what you want).

The further away from your DOF, the background (or foreground) is, the more out of focus it will be. So if you want your background to be really blurry, then try to use a shallower DOF and get the background as far away as possible. If you want it blurry, but not totally blurry...then deepen your DOF or get closer to it.

As for flash, I think the first step should be to determine the DOF that you want/need. Then match your flash output to give you the flash exposure that you want. Of course, a smaller aperture will require more flash output and you may find that your flash only has so much working distance when you're using smaller apertures. Raising the ISO will help.
The shutter speed will control the ambient exposure, just like always, but won't affect the flash exposure, so you can use it to help get the balance that you want. But beware, you won't want to exceed the camera's max flash sync speed (usually 1/250 or 1/200).

Typically, the lower the flash power, the better. That will give you faster recycle times and longer battery life. But you probably don't want to sacrifice your exposure or DOF, just for faster recycle times.

 

[KmH]

Another curveball seems to have been thrown my way when it comes to using flash, and aperture to control flash exposure. Should I be choosing the aperture I want to be working with first, and then playing with the distance between the subject and flash? I understand there can be quite a variance in distance - should I be aiming for the 1/1 power output range on the flash, or reduce the power (i.e. 1/4, or 1/8).

There is no one recommended aperture for doing portraiture, though most good portraiture is done at about f/8, because somewhere there around f/8 is the sharpest focus for most lenses.

Hot shoe flash units have a GN (guide number) number. Using the GN number of your flash unit - GN = distance × f-number. In this case the GN would be the power level you need to set the flash unit to to get a decent exposure with a specific subject distance and f-number.

By doing some basic algebra the equation can be re-written to solve for the lens aperture rather than the GN - f-number = GN ÷ distance.
Or we can solve for distance for a specific GN and f-number - distance = GN ÷ f-number.

Guide number - Wikipedia, the free encyclopedia

 

[Josh66]

The best resource you can have is first hand experience.

We could tell you "f/8 and don't be late", or you could learn it your self pretty quickly.

Practice a lot, and pay attention to what works and what doesn't.

 

[jwbryson1]

Big Mike wrote:

"The shutter speed will control the ambient exposure, just like always, but won't affect the flash exposure, so you can use it to help get the balance that you want. But beware, you won't want to exceed the camera's max flash sync speed (usually 1/250 or 1/200)."


Mike, can you please explain why this is so? It is counterintuitive to me that the light from a speedlight is not somehow "picked up" when the shutter activates, and only the ambient exposure is affected by the shutter speed.

Thanks.

 

[Trever1t]

because the speedlight "flash" is only measured in thousands (or less) of a second. SO as long as the shutter is open say 1/800th (for example) it will allow all the light from the flash to reach the sensor/film

 

[kirbym2]

Appreciate the info and links! A couple of follow ups...
1) Zoom-head position relative to focal length of lens. My SB-600 tops out at 85mm... I was told to set this at 1.5x focal length of the lens. What if I'm shooting with something longer than 60mm?

2) GN... is this established at iso 100? Example for me... subject at 10ft, determine aperture based on calculation of 98 (GN for SB 600)/10 = aperture of 9.8 (so f/8 or f/11)? What if iso sensitivity is at 200, 400?

Thanks!

 

[Josh66]

GN is usually (always, from what I have seen, but I haven't seen everything - so I'll just say 'usually') at ISO 100. If your working ISO is different than that, it is only a matter of doubling or halving the figures by how ever many stops off the working ISO is compared to the GN ISO.



w00t!
My 10,000th post!

 

[clanthar]

Big Mike wrote:

"The shutter speed will control the ambient exposure, just like always, but won't affect the flash exposure, so you can use it to help get the balance that you want. But beware, you won't want to exceed the camera's max flash sync speed (usually 1/250 or 1/200)."


Mike, can you please explain why this is so? It is counterintuitive to me that the light from a speedlight is not somehow "picked up" when the shutter activates, and only the ambient exposure is affected by the shutter speed.

Thanks.

When we first think about using flash it's because there isn't enough ambient light to get an exposure of a moving subject (people). You could always drop the shutter speed -- lets say 1/4 second would give you a good ambient light exposure at f/8. You don't want the blur that would come with a 1/4 second exposure (people move and you move). So you raise the shutter speed to 1/125 sec. If you take that photo you have a 5 stop underexposure for the ambient light -- you have nothing. Add flash and you have a flash exposure determine only by the f/stop and flash power/distance to subject. Add the flash exposure and ambient light exposure together and you get the flash exposure. 1 + 0 = 1.

But lets say you take the photo at 1/4 second with the flash. You now have a double exposure. The flash exposure happens in say 1/2000 second. When the flash exposure is over what percentage of the ambient light exposure has completed? If the ambient light exposure records now you may get something like this: http://userfiles.the-digital-picture.com/CommunityServer.Components.UserFiles/00/00/00/31/80/IMG_2875.jpg?AWSAccessKeyId=1TP5FAW3YKZH7C6FXJR2&Expires=1315883385&Signature=AulR5z0srKyBQwnFtPtweOpcEwI%3d

(I searched Google images). That's a double exposure. The flash exposure freezes action and the ambient light exposure is blurred. In this situation you have to be more careful determining exposure. If the ambient light is going to blur then you really have two exposures that are out-of-sync. If the ambient light exposure isn't blurred and the subject hasn't moved much then you get an overexposure 1 + 1 = 2.

The photographer needs to stay on top of the flash ambient light ratio. If you know the ambient light exposure is 3 plus stops below the flash then you're taking a flash photo. If the ambient light exposure is closer to the flash exposure or if that's what you want then you have to take responsibility for both exposures.

1. ISO + f/stop + flash power/distance
2. ISO + f/stop + ambient light + shutter speed.

Joe

 

[analog.universe]

Also look up some tests for the lens you're using and take some test shots yourself. It's true that most lenses are sharpest at f/5.6-f/8, but that doesn't mean that they're not plenty sharp enough at 2.5 or 11. Some lenses don't ever seem to flinch, and others are very particular about what aperture they're shot at. Get to know your gear and don't just assume you'll have lower quality results because you're not at f/8.

 

[KmH]

The GN number a flash maker claims is always stated in such a way as to make it seem as large as possible, so you have to pay attention to how it is defined.

For instance, Canon inflates their speedlight GN numbers by stating them at the longest zoom the flash is capable of, not at the industry standard of 50 mm.

Speedlite 580EX II

Guide Number - 190 (at ISO 100, feet) / 58 (meters); flash head at 105mm zoom setting

(my emphasis)

Here is how Nikon states it for their top-of-the-line SB-900:

Guide Number
34m/111.5ft. (at ISO 100, 35mm zoom head position, in FX format, standard illumination pattern, 20°C/68°F) to 48m/157.5ft. (at ISO 200, 35mm zoom head position, in FX format, standard illumination pattern, 20°C/68°F)

 

[kirbym2]

Great info all around. I see that the GN can be a good resource, but it's left me a little confused. As far as the calculation I want to use the following example.

Assuming a distance from subject 10 feet (reasonable working space outdoors), aperture of 8 (working with a consensus of what it takes to get ideal sharpness, DoF, etc). Based on this, GN should be 80? Using the table that comes in the manual for the SB600, assuming the flash head is at 50mm, I should be setting the flash to 1/2 power?

Reference: SB600 Manual pg. 35 (for the table)

Am I on the right track here? Math and equations were never my strong suit. I just want to make sure I have the variables correct.

Also, if I set aperture correct for flash exposure, will this freeze the subject regardless of having a slower shutter speed to capture more ambient light? I'm thinking for photos taken in the later afternoon/early evening.

Thanks for putting up with me... you guys have been amazing!

 

[clanthar]

Also, if I set aperture correct for flash exposure, will this freeze the subject regardless of having a slower shutter speed to capture more ambient light? I'm thinking for photos taken in the later afternoon/early evening.

Flash freezes action. Typical speedlite type flash units have burn times of 1/1000 sec. or less which is sufficient to freeze most people; maybe not Usain Bolt.

However if an ambient exposure is recorded as well it could be blurry. In which case you'd see both exposures overlayed on top of each other -- the sharp flash exposure and the blurry ambient exposure.

Joe

 

[KmH]

From page 88 of the SB-600 user's manual;

Flash Duration:

1/900 sec. at M1/1 (full) output
1/1600 sec. at M1/2 output
1/3400 sec. at M1/4 output
1/6600 sec. at M1/8 output
1/11100 sec. at M1/16 output
1/20000 sec. at M1/32 output
1/25000 sec. at M1/64 output

Using less than full power also lets the flash unit re-cycle (re-charge) faster.

When I'm setting up a shot or series of shots, I'm almost always doing some photography math in my head as I set it up.

Flash can be sync'd to either the front or the rear shutter curtain. i recommend syncing on the rear curtain. For the Canon shooters out there, Canon calls them the first and second curtains. Sync to the second curtain.