DOF & 18-250mm lens

[Learn2C]

Hello,

Newbie here who is struggling with Depth of field and my lens. I have studied aperature and understand how it relates to depth of field but I cannot seem to be successful with my lens. I have a Pentax K20D with a SMC Pentax DA 18-250 mm lens. F 3.5-6.3.

My question is, am I able to get a shallow depth of field photo with this lens. I have tried numerous times with different distances from the subject as well as different focal lengths and I just do not get it!

I have an old manual Pentax SMC 1:2 50mm lens f 2.0 - 22 and I have no problem getting nice shallow depth of field shots.

I have read where the closer you are to your subject the more shallow depth of field you get and the farther away from the subject gets you greater dof. I have tried many possible combinations with the 18-250mm lens and am not successful............. Would I be better off to get a different type of lens ( with a smaller aperature) for these types of shots?

ANY advice would be greatly appreciated at this point. Hope this all makes sense.

Carrie

[Izzy]

No matter the distance from your subject you will never get the same separation/DOF of your faster (f2.8 I think it was). It is not until f2.8-3.0 that you get that separation you are looking for. 3.5 just does not cut it.

In short you will be in the market for a new lens if you want that effect and it will not come cheap. You also will not get that same zoom range, you may get 70-200 but that is about as good as it gets. Super zooms never have a great DOF effect.

[Learn2C]

Thank you Izzy for your thoughts. The reason I went with this lens was because I was told it was a good 'all around' lens for a beginner. I have found out that I like to be able to get shallow dof in my photos, so the journey begins.

In your honest opinion, would I be better to stay away from another zoom lens ( 70-200) and just stick with a wide angle or standard lens? I do not know much about lenses at all so bare with me Any recommendations.. OOPS, that might be a different thread.

Thanks again,

Carrie

[lon gauthier]

you could attain a similar look in photo shop, by making a duplicate layer then using a blur filter. After the top layer is blurred, you can use the eraser tool to remove the blurred layer.
this exposes the sharply focused layer underneath. With a little practice this method is a very good one. It's definitely cheaper than a new lens.

[Izzy]

Originally Posted by Learn2C

Thank you Izzy for your thoughts. The reason I went with this lens was because I was told it was a good 'all around' lens for a beginner. I have found out that I like to be able to get shallow dof in my photos, so the journey begins.

In your honest opinion, would I be better to stay away from another zoom lens ( 70-200) and just stick with a wide angle or standard lens? I do not know much about lenses at all so bare with me Any recommendations.. OOPS, that might be a different thread.

Thanks again,

Carrie

It depends on what you intend to shoot. You have to ask yourself if you really need that much zoom. Try turning the lens to about 50mm and keep it there for awhile. This will give you a rough idea of what to expect from a prime lens like a 50mm. Nothing wrong with a zoom. I have several. My favorite lens for obtaining the effect that you want is my 50mm 1.8 AKA nifty fifty. Many people here use them and are hard pressed to take them off their camera. I love mine. I just checked on ebay and there are tons of them for sale but you need to know what mount your lens is so you can select a lens that will fit it. I know NOTHING about Pentax gear. I am sure there are people here who are better versed in this brand. I would expect the lens to cost you about $150 tops but I could be wrong. I know they are about $100 for Canon and Nikon.

[jml79]

Originally Posted by Learn2C

Hello,

Newbie here who is struggling with Depth of field and my lens. I have studied aperature and understand how it relates to depth of field but I cannot seem to be successful with my lens. I have a Pentax K20D with a SMC Pentax DA 18-250 mm lens. F 3.5-6.3.

My question is, am I able to get a shallow depth of field photo with this lens. I have tried numerous times with different distances from the subject as well as different focal lengths and I just do not get it!

I have an old manual Pentax SMC 1:2 50mm lens f 2.0 - 22 and I have no problem getting nice shallow depth of field shots.

I have read where the closer you are to your subject the more shallow depth of field you get and the farther away from the subject gets you greater dof. I have tried many possible combinations with the 18-250mm lens and am not successful............. Would I be better off to get a different type of lens ( with a smaller aperature) for these types of shots?

ANY advice would be greatly appreciated at this point. Hope this all makes sense.

Carrie

Ah, Yes but there is more. I will try my best but someone may have to correct me.

What you said applies. Heres why. As you move away from the subject you approach the hyperfocal length and then infinity focus. At a short zoom this happens very quickly. At 35mm and 3m from your subject you will have over 1m depth of field and a hyperfocal distance of 12m. If you move closer the depth of field will shrink but not by as much as you want and the hyperfocal distance won't change.

But. If you change your focal length the whole equation gets changed. And it can change faster than your field of view does. As you go longer and move away from your subject your DOF will remain the same but the hyperfocal distance moves off rapidly. This can allow you have more background blur while having the same subject. At 200mm and 17m from your subject the subject framing will be similar but the hyperfocal distance will now be 397m away.

This difference between hyperfocal distance is what can make the same DOF appear to be shallower because what is not in focus is more not in focus than with a shorter focal length. In reality you have pulled your focus further away from infinity.

Hope this makes sense. For the geeks out there i did all of the calculations at f/5.6 with a 1.5 crop APS-C sensor.

[i speak in math]

You shouldn't have any problems getting images with a shallow depth of field with that lens. Set it to >100mm, be about 10 to 15ft from your subject with a background 30-50 behind him.

Its all about large aperture, and camera to subject, subject to background distance

I don't have that lens, but here's a shot taken at 75mm f/4



Or 63mm f/8



In the latter image, the subect and background are both closer, and I'm using a smaller aperture, but the ratio of camera to subject, subject to background is pretty similar: short distance to subject, long distance to background.

You can really accentuate the effect with lenses that have even larger apertures

This was with a 50mm @ f/2

[BexJarratt]

Originally Posted by jml79

What you said applies. Heres why. As you move away from the subject you approach the hyperfocal length and then infinity focus. At a short zoom this happens very quickly. At 35mm and 3m from your subject you will have over 1m depth of field and a hyperfocal distance of 12m. If you move closer the depth of field will shrink but not by as much as you want and the hyperfocal distance won't change.

But. If you change your focal length the whole equation gets changed. And it can change faster than your field of view does. As you go longer and move away from your subject your DOF will remain the same but the hyperfocal distance moves off rapidly. This can allow you have more background blur while having the same subject. At 200mm and 17m from your subject the subject framing will be similar but the hyperfocal distance will now be 397m away. .

Is that the same as moving your background further away?

[RichardTaylor]

It depends on what aperture, focal length & shooting distance.
Izzy is right in the fact that you will never get the super thin DOF that wide apertures can give you. However it is possible to get relatively shallow DOF.

I have a similar lens, Tamron 18-250 F3.5-F6.3.

Here are some recent examples where the DOF has been shallow enough to isolate the subject.

(1)

amera Canon EOS 40D
Exposure 0.001 sec (1/1250)
Aperture f/6.3
Focal Length 250 mm
ISO Speed 400
Exposure Bias -2/3 EV
Flash Off, Did not fire

(2) If you are real close (minimum focussing distance) then even F10 may do the job.

Camera Canon EOS 40D
Exposure 0.003 sec (1/400)
Aperture f/10.0
Focal Length 250 mm
ISO Speed 800
Exposure Bias 0 EV
Flash Off, Did not fire

(3)

Camera Canon EOS 40D
Exposure 0.002 sec (1/500)
Aperture f/7.1
Focal Length 250 mm
ISO Speed 800
Exposure Bias 0 EV

(4) Here's another example but with a different lens @180mm & F6.3



Camera Canon EOS 40D
Exposure 0.001 sec (1/1000)
Aperture f/6.3
Focal Length 180 mm
ISO Speed 100
Exposure Bias 0 EV


The bottom line is to try shooting wide open, especially at the longer focal lengths, try to get relatively close to your subject and try to have the background at a distance.

Experiment with different focal lengths, apertures, and shooting distances, and background distances.

I wouldn't buy any more lenses until you have mastered the one you have now.

[Learn2C]

Thank you Richard for sharing your photos along with the EXIF data. I am sure I have tried every possible combination to no avail. I will look at photos in detail and try some similar combinations. Don't know what else to do at this point other than to get a different lens that will allow me to get the results I am looking for without the 'headache'.

As I mentioned in my original post, I had no problem getting the shallow dof in my photos with my old manual 50mm f 2.0 lens.

The hyperfocal discussion went right over my head! Hopefully I do not have to know this in great detail or I am sunk.

Many thanks,
Carrie