Zooming VS F/stop; How to Lower DOF for Portraits?

[DGMPhotography]

So I've been reading up on portrait stuff and I have a question. From what I've read you're supposed to have a focal length no lower than 50, but preferably around 80 or so. Then, you're ALSO supposed to have a low f/stop so that you achieve the desired bokeh/blurred background effect.

Now then, with my kit lens. I can zoom all the way in to 55mm. However, that makes my minimum f/stop 5.6. Zooming in raises f/stop so wouldn't that be counter-intuitive to getting bokeh. How are you supposed to both zoom in, and lower f/stop??

Maybe this is a dumb question, but I just can't figure it out.

Thanks.

 

[480sparky]

So I've been reading up on portrait stuff and I have a question. From what I've read you're supposed to have a focal length no lower than 50, but preferably around 80 or so. Then, you're ALSO supposed to have a low f/stop so that you achieve the desired bokeh/blurred background effect.

Now then, with my kit lens. I can zoom all the way in to 55mm. However, that makes my minimum f/stop 5.6. Zooming in raises f/stop so wouldn't that be counter-intuitive to getting bokeh. How are you supposed to both zoom in, and lower f/stop??

Maybe this is a dumb question, but I just can't figure it out.

Thanks.

Your lens only changes the f/stop when you're shooting 'wide open'. If your kit lens, say, is the 18-55, at 18 it's at f/3.5, at 55 it's at f/5.6. If you set the aperture to f/5.6, it will stay at 5.6 all they way through the zoom range. If you set the aperture to f/8, it will stay at f/8, whether the zoom is at 18mm or 55mm.

 

[Buckster]

Portrait photographers prefer to use lenses that are made with wider apertures, and generally, they prefer primes, like an 85mm f1.8 or f1.4 or 50mm f1.8 or f1.4, or even f/1.2 if they can afford it, depending on the camera format (50mm on aps-c, 85 on full frames). If you don't have a lens with a wider aperture, you're forced to put more distance between your subject and the background to achieve more blur via DOF.

Some folks prefer even longer lenses, if they can get their distance to subject. 100mm, 130mm, 180mm and more all work. With longer lenses, you can get away with f/4.5 or f/5.6 and still get a nice blurred background but, again, you've got to have the distance to subject so they'll fit in the photo.

As for the over-misused term "bokeh", it refers to the QUALITY of the blur, not how much blur there is. Lenses with more (8 or more) aperture blades make rounder, softer blur, and that's usually seen as "good bokeh". Lenses with few (5) blades tend to make 5-sided, jittery-looking blur, which is usually seen as "bad bokeh".

 

[KmH]

Several factors determine the DoF:

• Image sensor size - The bigger the image sensor, the shallower the DoF with everything else equal.
• Focus point distance - The closer the focus point distance, the shallower the DoF
• Lens focal length - Longer focal lengths magnify burred background elements making them seem blurrier.
• Lens aperture - DoF gets shallower with wider lens apertures.

How far your subject is from the background determines if the background is inside or outside the DoF. If your subject is leaning against a wall, don't expect the wall to be blurred much, if any.

DoF, particularly when a shallow DoF is wanted, is probably the single most difficult concept new photographers have to understand.

 

[TCampbell]

Sparky, Buckster, and Keith have you fixed up.

Basically a variable focal-ratio zoom (most "kit" lenses) fight themselves. I hesitate to say you need a "long" focal length... mostly what you need is to stay away from wide angle. Anything from a normal through long focal lengths work (and the longer the better, but even "normal" is adequate.) BUT... when you get to that normal to long focal length you need an aperture that does not ALSO get small at the same time and, unfortunately, that's what most kit zooms do (as Sparky points out above.)

While you COULD buy a zoom that offers a constant f/2.8 focal ratio, you'll find those are fairly expensive. Instead, get a prime lens. A 50mm f/1.4 lens would be perfect. 85mm would be even better BUT... you'll find you have to walk back farther to get your subject in the frame. If this is for outdoor use it's no biggie. But in a studio the room may not be long enough when used on a crop-frame camera body such as your D5100.

You would get the effect on the 50mm f/1.8 too, the difference is the 1.8 has 7 aperture blades and the 1.4 has 9 -- providing a more "rounded" aperture for better quality bokeh.

f/1.4 is four full stops of more light than f/5.6 -- so you'll also find the lens is more versatile with lighting as well. You may find that you prefer to shoot at about f/2... and sometimes f/2.8 vs. wide open at f/1.4.

 

[DGMPhotography]

Hmm, okay, so it's the kit lens that fights itself? There are lenses that can zoom without the aperture changing? And why would you prefer f/2 vs f/1.4? You can still change the f/stop with a prime right?

 

[Patrice]

All these "supposed to ..." that you've been reading about are general guidelines and popular opinion, they are not rules you must follow. Learn to use the equipment you have, learn about what things mean, develop a style or approach that works with what you have.

A forum name like DGMPhotography would lead one to believe you already have a pretty good idea of what you're doing. Keep working what you have and learn and progress, soon you'll have grown into the name.

Good luck.

 

[o hey tyler]

Hmm, okay, so it's the kit lens that fights itself? There are lenses that can zoom without the aperture changing? And why would you prefer f/2 vs f/1.4? You can still change the f/stop with a prime right?

Yes, you can change the aperture with a prime lens. What he's referring to is shooting at f/2 rather than wide open for a large bump in sharpness. I have 3 f:1.4 primes (35, 50, 85) and they're all shot at f/2 or smaller for adequate sharpness.

 

[480sparky]

Hmm, okay, so it's the kit lens that fights itself?

Not really. You still aren't understanding.

There are lenses that can zoom without the aperture changing?

Yes..... yours included. Set the aperture at f/5.6 or smaller, and it will not change during zoom. It's only when the lens needs to open the aperture to the max will it change.

And why would you prefer f/2 vs f/1.4?

I'd love to see a tele-zoom that's f/1.4. F/1.4 would be great for focusing, etc. But I wouldn't want to shoot at that setting.

You can still change the f/stop with a prime right?

Yep.

 

[TCampbell]

The background blur effect is enhanced by:

1) Low focal ratios (low f-stop values)
2) Long (high focal length values)
3) Closer subject distance and farther background distance.

The problem with the kit lens is that as the focal length gets LONGER (which should improve the background blur) the minimum f-stop gets higher... so high that it's not longer good for a pleasantly blurred background blur.

When you use a low focal length then you can't attain the effect EVEN with a low-focal ratio. For example... I have a 14mm wide angle lens with an f/2.8 focal ratio. Normally f/2.8 is great for background blur EXCEPT at the 14mm length it just wont work. I can focus my 14mm lens to about 3' and even at f/2.8 I will get absolutely NO background blur at all.

As for zoon lenses that can maintain a low focal ratio all throughout the range, yes they make them BUT they are expensive.

For example: AF-S DX Zoom-Nikkor 17-55mm f/2.8G IF-ED from Nikon

If you think the focal length range on that zoom is astonishingly close to your kit zoom... you'd be right. EXCEPT that lens can maintain an f/2.8 focal ratio at all focal lengths. But notice it's $1500+ price tag.

You can pick up a 50mm f/1.4 lens for less than a third of the price (nearly 1/4 of the price).

The reason I don't normally suggest shooting all the way down at f/1.4 is for two ... ok three reasons.

#1 At a 50mm focal length you'll find the effect is already pleasant at f/2.8 and it'll be great at f/2.

#2 Most lenses don't yield their best work at "wide open". The image quality at f/2 will be better than the image quality at f/1.4 (even though the intensity of the background blur will be less.)

#3 When you get to very low focal ratios (e.g. f/1.4) the depth of field starts to become extremely thin... so thin that your subjects face wont entirely be focused. You may notice their eyes are focused sharply but their nose and ears are actually slightly soft (or extremely soft depending on how close they are.)

Probably you will love the look at f/2, you'll be able to control the depth of field (depending on the subject distance... you can use a site like DOFmaster.com to calculate the depth of field) and the image quality will be better than it is at wide open. When I use 100mm lenses, I often use f/4 rather than f/2.8.

 

[o hey tyler]

As Tim said in many more words than I am about to...

F/1.4 prime lens @ f/2. You can't go wrong.

 

[KmH]

Many fast prime lenses having a maximum aperture - like f/1.2, f/1.4, f/1.8, or f/2 - use more of the lens diameter at maximum aperture than kit lenses do.

The way a simple lens is made, the middle of the lens is thicker and tapers until thinner at the edge of the lens.
Light that goes through the thinner edge area of the lens, focuses the light at a different distance than the middle part of the lens. That prevents the light from focusing with maximum sharpness. Lens (optics) - Wikipedia, the free encyclopedia

Camera lens makers add additional lenses right behind the front lens to correct that, and a number of other optical aberrations. But, when used wide open light through the thick and thin parts of the lens still focuses at very slightly different distances.
To mitigate that, we a 'stop down' fast lenses so less of the thinner edge area is used. Stopping down 2 stop is usually recommended if you want the sharpest focus the lens can deliver.
So a f/1.2 lens would be 'stopped down' 2 stops to f/2.5.
A f/1.4 lens would be 'stopped down' 2 stops to f/2.8.
A f/1.8 lens would be 'stopped down' 2 stops to f/3.5.
And a f/2 lens would be 'stopped down' 2 stops to f/4.

DoF can get so shallow that inexperience photographers have difficulty understanding how very shallow (think razor thin) the zone of acceptably sharp focus is.
In addition they don't understand how the DoF is distributed in front of and behind the point of focus.

Online DoF calculators usually describe/define the DoF distribution. Online Depth of Field Calculator
Understanding Depth of Field in Photography

Coming to grips with DoF requires some abstract thought, and an ability to make mental pictures of how DoF works also helps a great deal.

 

[DGMPhotography]

Thanks for the tips. So then if I want the sharpest photos with my lens which is 18-55mm f/3.5-5.6 I would need to have the f/stop at at least 4.5 at all times to get maximum sharpness? To get bokeh with my kit lens I would need to zoom into 55mm to get the least distortion, etc., my f/stop would be at 5.6 and so I would have to have some distance between my target and the background which I can calculate using the DOF calculator?

 

[KmH]

Not necessarily, because your kit lens isn't 'fast' and doesn't use much, if any, of the thin edge area.
Bokeh is about the visual quality of an out-of-focus area in a photo.
DoF is about how out-of-focus that area is, and how deep the acceptably sharp focus area is.

You are using the term 'bokeh' in the wrong context. You are asking about depth-of-field, not bokeh. Zooming, lens aperture, focal length. focus point distance - have zero effect on the bokeh any lens make/model produce.
The only way to change bokeh is to get a different lens.
Bokeh is not adjustable. The bokeh you get is determined by the lens design and construction.

Your $120, 18-55 mm lens has 7 lens elements in 5 groups. It's very likely some of those lens elements are made from resin rather than glass. the front lens element is an aspherical lens and the 18-55 mm has 1 ED (Extra-low Dispersion) glass element. Your 18-55 mm has 7 aperture blades.

By comparison, the $1540 Nikon 17-55 mm f/2.8 lens has 14 lens elements in 14 groups. 3 lens elements are aspherical and 3 are ED glass. The 17-55 f/2.8 has 9 aperture blades.

Either of those 2 lenses mounted on your camera - using the same focal length, lens aperture, and focus distance - will deliver the exact same DoF. But the $1540 17-55 will deliver 'smoother' bokeh because it uses better glass, and has more aperture blades.

You can visit NikonUSA.com and see the lens construction, specifications, and MTF charts for each lens they currently offer. The web site also has a Glossary so you can gain further insight into how things like aspherical and extra-low dispersion lens elements are beneficial.

 

[480sparky]

If you really want good bokeh, get the 85mm Nikkor.... AKA The Cream Machine.